Late game 3000+ science pack fusion or antimatter power supply that needs copious water supplied, and can only be built with assemblers. Something power dense like 1MW each in a 3x3 space and zero pollution. This would keep the space requirements down for power supply, and reduce the pollution ...

At max level pollution and max sized hive, I'd like an omega level/gigantic sized alien that just walks through walls and needs 4+ max bonus'd gun turrets, or 3+ max lasers, or 2+ flame throwers to take it down. It'd be a nice incentive to keep your local area clean and keep the pollution level down ...

Post tank and prior to power armor Mk2, a hover craft capable of going over water but bouncing off of trees with lasers plus machine gun ammo would be sweet. Maybe a few less HP but able to escape over water to allow repair bots to fix your busted boat.