If you'll give me the source code I would like to make a few additions to the game. Namely, "focused aim", motion sensor, and proximity mines. Add few more sound effects as well.
Focused aim: pressing a wait key while targeting would increase chances for critical hit by %50-%100, in exchange for skipping a turn.
Motion tracker: blink symbols '?' at invisible alien locations once each turn accompanied by the sound effect which gets more intense (higher pitch) the closer they are. Perhaps aliens in vents would only give off sound effect without visual clue, which could work well to add some more tension to the game. I guess tracker should need to be equipped instead of primary or secondary weapon to be used.
Proximity mine: not to throw, only to lay down at your current location. Activates when aliens step on it.
The only thing I don't like about the game is that most of the time you end up shooting at aliens when they are right next to you, for several turns while being clawed by them in the same time. I think you should be able to deal with one and two aliens before they get close in most situations. The real danger would be using inadequate fire power, getting yourself surrounded or attacked by more than few aliens at once. Addition of "focused aim" should move the balance a bit more towards what I described. It would give you a chance to finish off a single target at greater distance, but at the cost of all the other aliens moving in closer. Additionally perhaps aliens HP could be less in exchange for their increased numbers and/or dealing more damage when they do get close.