Wrote this up last night. Felt like this question needed an answer... not exhaustive should answer the basics.
First, I have to explain what its like there currently. HK controls the Western half. Order obsessed and expansionist. Their main problem is that they require lots of control over all their lands or they decentralize and then sections will split off them into smaller Civs. Disloyalty when your a walled settlements is easier, than when your unwalled and they've been fortifying. At the same time they've got it in their collective heads they need to expand. I think they evolved a value to cause that. They are current involved in a two front war (that we know of South is all vague and map fuzzy to The People).
The second, major faction is the Xoh. The Xoh are the control center of the eastern lowlands. A city vile in hygiene and governmental values. A city thay needs a constant influx of people for replacements for generics (toxic hygiene for the low classes) and sacrifices to keep it moving and working. The main reason they haven't been wiped out is that they are both in the middle of nowhere and important as a staging area to try and control the lowlands.
Third, the Xoh are currently in charge (personal union and marriage alliance silliness) of the empire-like-thing known as the Thunder Horse. Its got at least 6 or 7 vassal states involved last check (Xoh, TS, Swamp People, Thunder Horse... guys we haven't got info on to the east.)
Currently, the Thunder Speakers are revolting for a second time (vile Xoh contaminating the rulers with their Xoh-ness) and with Xoh so hellbent on the feeding the lowland meat grinder... they may actually break off. Only they seem to have assimilated enough they are invested in that TH thing so don't really want to break off. So its more a matter of hating what Xoh assimilation has done to TH and feeling the need to purge Xoh influence if not Xoh itself.
That established we go into scenarios the near future of the lowlands. Only some of them end up with land taking in the Lowlands by the People being a real option.
Scenario 1: Xoh stomps the TS with the rest of TH as a bludgeon. HK vs TH stalls.
This takes pressure off the HK's eastern front so they can concentrate on the Hats. HK got resources and fortifications out of this so any land they can hold is just icing on that front. A stalemate means that more population goes into the black hole to the NE and the Hats can't deal with that status effect forever. Entire walled settlements are now ghost towns from this effect during this mess.
The Hats are going to have issues just getting land they once had back in this scenario. A lot of this is due to that great siphon to the NE the Hats actively need to actually win or at least stalemate HK and stabilize or the population votes with it feet and they run out of people to rule. What is left is a matter of what is far away enough the population can't be siphoned off and HK feels they are too over extended to handle.
Scenario 2: Xoh stomps the TS with the rest of the TH as a bludgeon. HK vs. TH = HK winning
HK enters 'happy fun roll saving throws versus over extension with negatives from the multi front war' mode. HK gets a turn of expansion relatively unstopped and then the forces start leaving the TS zone and HK probably has to cede progress to keep the core and near core areas on this front.
Hats have a better chance of stabilizing or taking back land here as HK is spreading things out more. If the Hats can advance this means that HK is the one that starts losing population to the vampiric population singularity to the North. This occurs and HK gets one hell of a chance of stability drops making things worse for them.
Expansion for the thread is the Northern most section of Hk splintering and joining the Hat River March. mostly hills on the edge of the lowlands. May inherit the Hats ire. May have the Hats head towards the capital of HK. May be worth it to relay pass that section of western war front if HK gets screwed enough.
Scenario 3: Xoh stalemates with TS and its double meat grinder time for Xoh's TH.
Hats vs HK war already covered the results of the HK front enough for reference. So its a matter of if the Xoh can stalemate the Lowlands front, is losing on the lowlands front, or somehow is wining on the Lowlands front.
Losing to HK on the Lowlands front means HK plays chicken with over extension. Stalemating means that the TS get to be under less pressure and could make gains or make it a defensive war for themselves. Two meat grinders and one less resource base would be bad for TH. The longer this goes on the less people want Xoh to be in charge.
Scenario 4: Xoh TH starts losing to TS.
This is where everything goes to hell in a hand basket in the Lowlands. HK goes for the over extension collapse achievement. Xoh risks being burnt out. TH risks civil wars for generations once the old nobles are dead. If Xoh is actually burned there is no center of control in the Eastern Lowlands. This means either trying to run things from the TS or Swamp People's lands or building an entirely new control center.
---
In short: the only time its really worth expanding into the lowlands is if HK shatters. The window is the time period they are shattered. Xoh burning really aids this happening as would a Hat counter strike deep into HK lands.
Even then the better extension is taking in the North most part of HK and solidifying that, then going SW in (ex)Hat lands.
Its because the lowlands currently aren't worth it. Too much land. Too many competing factions. Too many disparate values. The random settlements don't want or need to assimilate as your just 'Meet he new boss... ... same as the old boss... ...' too them. They just want to know what tribute your demanding so they can get back to day to day life. So what has to change?
First, I have to explain what its like there currently. HK controls the Western half. Order obsessed and expansionist. Their main problem is that they require lots of control over all their lands or they decentralize and then sections will split off them into smaller Civs. Disloyalty when your a walled settlements is easier, than when your unwalled and they've been fortifying. At the same time they've got it in their collective heads they need to expand. I think they evolved a value to cause that. They are current involved in a two front war (that we know of South is all vague and map fuzzy to The People).
The second, major faction is the Xoh. The Xoh are the control center of the eastern lowlands. A city vile in hygiene and governmental values. A city thay needs a constant influx of people for replacements for generics (toxic hygiene for the low classes) and sacrifices to keep it moving and working. The main reason they haven't been wiped out is that they are both in the middle of nowhere and important as a staging area to try and control the lowlands.
Third, the Xoh are currently in charge (personal union and marriage alliance silliness) of the empire-like-thing known as the Thunder Horse. Its got at least 6 or 7 vassal states involved last check (Xoh, TS, Swamp People, Thunder Horse... guys we haven't got info on to the east.)
Currently, the Thunder Speakers are revolting for a second time (vile Xoh contaminating the rulers with their Xoh-ness) and with Xoh so hellbent on the feeding the lowland meat grinder... they may actually break off. Only they seem to have assimilated enough they are invested in that TH thing so don't really want to break off. So its more a matter of hating what Xoh assimilation has done to TH and feeling the need to purge Xoh influence if not Xoh itself.
That established we go into scenarios the near future of the lowlands. Only some of them end up with land taking in the Lowlands by the People being a real option.
Scenario 1: Xoh stomps the TS with the rest of TH as a bludgeon. HK vs TH stalls.
This takes pressure off the HK's eastern front so they can concentrate on the Hats. HK got resources and fortifications out of this so any land they can hold is just icing on that front. A stalemate means that more population goes into the black hole to the NE and the Hats can't deal with that status effect forever. Entire walled settlements are now ghost towns from this effect during this mess.
The Hats are going to have issues just getting land they once had back in this scenario. A lot of this is due to that great siphon to the NE the Hats actively need to actually win or at least stalemate HK and stabilize or the population votes with it feet and they run out of people to rule. What is left is a matter of what is far away enough the population can't be siphoned off and HK feels they are too over extended to handle.
Scenario 2: Xoh stomps the TS with the rest of the TH as a bludgeon. HK vs. TH = HK winning
HK enters 'happy fun roll saving throws versus over extension with negatives from the multi front war' mode. HK gets a turn of expansion relatively unstopped and then the forces start leaving the TS zone and HK probably has to cede progress to keep the core and near core areas on this front.
Hats have a better chance of stabilizing or taking back land here as HK is spreading things out more. If the Hats can advance this means that HK is the one that starts losing population to the vampiric population singularity to the North. This occurs and HK gets one hell of a chance of stability drops making things worse for them.
Expansion for the thread is the Northern most section of Hk splintering and joining the Hat River March. mostly hills on the edge of the lowlands. May inherit the Hats ire. May have the Hats head towards the capital of HK. May be worth it to relay pass that section of western war front if HK gets screwed enough.
Scenario 3: Xoh stalemates with TS and its double meat grinder time for Xoh's TH.
Hats vs HK war already covered the results of the HK front enough for reference. So its a matter of if the Xoh can stalemate the Lowlands front, is losing on the lowlands front, or somehow is wining on the Lowlands front.
Losing to HK on the Lowlands front means HK plays chicken with over extension. Stalemating means that the TS get to be under less pressure and could make gains or make it a defensive war for themselves. Two meat grinders and one less resource base would be bad for TH. The longer this goes on the less people want Xoh to be in charge.
Scenario 4: Xoh TH starts losing to TS.
This is where everything goes to hell in a hand basket in the Lowlands. HK goes for the over extension collapse achievement. Xoh risks being burnt out. TH risks civil wars for generations once the old nobles are dead. If Xoh is actually burned there is no center of control in the Eastern Lowlands. This means either trying to run things from the TS or Swamp People's lands or building an entirely new control center.
---
In short: the only time its really worth expanding into the lowlands is if HK shatters. The window is the time period they are shattered. Xoh burning really aids this happening as would a Hat counter strike deep into HK lands.
Even then the better extension is taking in the North most part of HK and solidifying that, then going SW in (ex)Hat lands.
