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Ridiculous Ideas (to get them over with)
- Pick a side. Terrors or Predipups, perhaps, or Hive or Predipups. Depending on which side you pick, after you go through their quest sequence, you can wander around their areas freely, perhaps with your allies giving you heals or buffs sometimes, or attacking things you attack [invading enemies from the other side, and just natural no-side hazards]. You can also gain Ghi from some areas in it, just like Barton, and perhaps have some shops or other passive side stuff. For the other area, you have to fight your way through.
--The areas might be instanced according to which side you take. Thus, if you were hostile, you wouldn't run into people friendly with the animated.
--Alternatively, you could run into each other, but there would be some storyline stuff to explain why you aren't helping your allies (or getting reaaally angry at the people slaughtering them all).
--The craziest form would be PvP. Probably not actually attacking each other (unless there was a system set up for that - but I generally like the arena or ticket style forms of that, and those wouldn't seem to fit in too much) - but instead healing and buffing your allies (Predipups or whatever).

Okay, more sane ideas:
- Ancient ruins (not just to fit with that one idea Kutai mentioned earlier in this forum - ruins underground are cool).
- Sewer pipe animated earthworms?
- Several battleground areas which would regularly (somewhat randomly, somewhat time based) switch control between different factions (Hive, terrors, predipups, maybe even Gaian NPC's and maybe different factions of predipups). Quests might work to up the odds for one faction or the other to get their turn to shine (or just basic slaughtering of every single animated around might do so).
-Needing to get certain technology (achievable by random drops from predipups in various areas) to counteract environmental debuffs (getting alien-lantern would combat accuracy debuffs in dark areas, etc.).