So the suggestion is a new UFO, plain on the inside, not strong in battle and kinda big. It needs to be big, since it carries a (metric) ****ton of alien soldiers around...
More than a battleship, thought about 35-45.
The idea behind that is to have even lategame a really hard tactical battle that needs all your skills, even with good equipment.
Needs to be lategame though, since before lasers you´d run out of ammo and before researching plasma you´d have no chance if it accidentally contains mutons... ^^
Could be used on alien terror missions well, since there you´d at least have a lot of possibilities to take cover.