I think the collective wet-dream boils down to MechWalker vs. Flying Alien, which, if introduced together, could influence the Jetpack vs. Tyrant (Skybase/Ventbase vs. Too-Big, Non-Wallwalking, Lame-Jumping Lumbering Behemoth) type of endgame (on a map like Karith or Thermal or even ATCSzalpha). Both together, in one mod, with a lot of play-testing, and maybe initially limited map rotation, might be a cool mini-mod. Love/hate/indifference?
Humans as lurching berserkers and Aliens capable of flight? A new alternative at Sudden Death (new weapon/class unlocked and "buyable" for a goodly/major amount of creds or comparatively few evos)? Open the Sky Project meets the old Berserker idea? IDK, mebbe could be cool. The walker could have one of the following: rockets/nades/freezer/zapper/gaussgun/sniperrifle; and outside it the Human is naked with a Blaster. The flying alien would be a relatively cheap class evo-wise (2-3 evos), while the walker would be the equivalent of "buying" a Tyrant, like 1000-2000 credits.
Instead of a kills-based stage-up, it would be time-based (and therefore unearned in the hardliner/oldtimer school of thought) like Sudden Death or Rampage, but unlike Rampage each individual player still needs the credits or evos to get the upgrade (cool stuff), and unlike Sudden Death it doesn't affect structures or rebuilding. An endgame variant, time-based, but still with a cost in credits or evos to participate, that doesn't affect buildings, and could be combined with SD, or not, as the ServerOp sees fit.
Also, the walker would be a field pack which the Human would have to deploy beyond his own buildzone; in other words, you have to take it away from base and "unfold" it to get inside. You can't carry another weapon besides your blaster, and you can't wear any armor (well, maybe a helmet and light armor, but I'd rather that your team has to support your lone berserker scheme or that you have to be skilled and lucky to use it).
The flying alien would either not be available to evo-down to or would only be available at respawn, limiting its use, likewise.