I agree that after this we go for some actual villains but I don't see why we can't make Theo a Master too? He'd only be our third, so four villains against two heroes and Theo somewhere in the middle is alright to me.
I prefer rogues rather than villains here. Looking at the choices, we still have ABB, Merchants, Faultline, Undersiders, and several other rogue capes. Also dont forget Amy. Most of them are more rogue than villain.
If its the ABB, I would prefer Oni Lee to be the master. His kinda a blank here and it would make some interesting situations. He might have a souchirou/medea kind of relationship with his servant. Perhaps summoning Altera, due to how compatible they are. He might even be influence by servants like Gilgamesh. Fun options.
Merchants, well, um, pass.
Faultline though. Whoo. Out of everyone, they have the most clear and definite wish. Uncovering the truth of the case-53 or for Labyrinth, finally free from her powers. As for servants, I think 'monster' servants would be the most appropriate summon here. Asterios, Frankenstein, Vlad (Berserk). A wide range of options. Also, out of all the participants, I would think that Faultline group would be the one to clash the most with thr PRT. Cauldron definitely dont want the truth to get out and will push the PRT to stop them. This will cause some big conflict, which the HG and Angry person wants.
Undersiders or in this case Tattletale would also be a good participant, though I could see her being the one to be actively against us. Tattletale powers could let her now Taylors true nature and she would do anything to ensure that thr wish is not granted. Itll be annoying for the HG, but itll make for some interesting conflict. As for servants, like I said pretty early on, Moriarty or Sherlock. They would be quite fitting for her.
Dinah should be the same as Lisa. However, her servants choice would definitely be different. Less detective, more bodyguards. Herakles, Kintoki or NR would be a good choice for her.
There is also Coil, but seeing how people generally hate him, him getting chosen is pretty low.
Other rogues, such as U&L or Parian is there too. But I dont see them having any definite wish and they're pretty passive, which is something that the Grail dont want.
Finally, the two people that could be easily corrupted in this war, Amy and Danny. We already seen her potential for corruption and Danny would practically do anything for Taylor. They have the potential to be the big bad in this war and its only fitting to give them a servant suited for said role. We already talked a lot about Amy's servant, so Ill pass on that for now. Danny though, I think the most suitable servant for him would be a servant that is absolutely loyal to him, for example Karna. Said servant will follow him no matter what, throughout his journey of desperation and tragedy. A tragic master and a loyal servant, thats a combination that would make an interesting big bad.
