After thinking about the options a little bit, I want to suggest a new tileset:
It would be cool if you could add a terror mission on a cruise or/and military ship. As the game will go forward the dimensions of the terrorizing ufos would increase, so you could start with small 1 level boats terrorized (by scout crews?) in the beginning of the game, and finish with large, 3 level ships(what is the engine level limitations?) terrorized by large ufo crews (cruisers?, etc…)
Advantages-
1. You can design the ships in a way that it would be impossible to use tanks/large units (at least in the smaller ships), so it would be infantry on infantry battles, which would give some artificial variety(on planning and tactics) to terror missions.
2. You could reuse the tiles from the smaller ships on the larger ones, so it would be easier and faster to develop the whole ship tileset family. Basically, it would feel like you added a lot of content to the game (with less effort on your side).
The only disadvantages that I see is that you’ll have to make some water tiles (Possibly animated?) which could be hard with the current engine.
Sorry for the wall of text, English is not my native language.
Thanks for your efforts.