I am talking about big ideas, not that piddly stuff. If the sequel is underwater like Terror was, it is pretty similar to adding a double-barreled shotgun to Doom and calling it a sequel.
Yeah, Terror from the Deep was a bit silly. It really just felt as if they rehashed everything with different textures. The underwater combat was just like regular combat. You could even throw grenades. Under the water. That killed the immersion for me.
The problem is mainly one of lore/story I think. It would be pretty easy to do a sequel with more missions, new gameplay features, enemies, graphics etc. But how would you fit it into the X-com world? The charm of X-com is to gradually reveal the alien conspiracy and their technology. After the first game mankind already has access to all that. Then you either have to set the game far into the future (like some later orginal X-com sequels) or try to invent some excuse for a new storyline (TFTD). Or maybe reset the knowledge gained through some convoluted story twist. They all kinda suck.