We could actually do something like that, but it would take some research. Maybe it's something we could assign Scholarium researchers to?
To do it, we would first take the basic
Mimic and add the
Creature Swarm template. That would actually reduce them from CR 4 to CR 3, since it the Mimics would be losing their Constrict special attack. This would turn each Mimic into a
Swarm composed of
10,000 Fine-sized micro-Mimics.
Then we add the
Collective Creature template to bring them back up to CR 4. Among other things, this gives the Mimic Swarm the ability to combine with other Mimic Swarms to form a single larger Mimic entity.
Rather than Wood Element Creature, which wouldn't work well with this setup, I would then add
Fungal for the attribute bonuses, poisonous spores, and Plant immunities, along with
Advanced as usual.
Each
Advanced Fungal Mimic Collective Swarm would be CR 6. Each of them could operate as an independent Mimic Swarm as needed, but they could combine to form larger and larger Collective Mimics. Each additional Mimic Swarm added to the Collective would increase its CR by +1 and its HD by +2. Sticking with the CR 15 limit for most Forge creatures, I would say that no more than 10 Mimic Swarms could combine to form a single CR 15, 22 HD Collective Mimic.
It's not exactly a Shoggoth, but it's still pretty impressive. The Mimic Swarms would retain their ability to mimic objects, too, so a door, chest, couch, etc., could suddenly burst apart into a Swarm composed of tens of thousands of micro-Mimics.