Personally, I wouldn't mind shelling out $25 for an updated Xenonauts 1 provided the following is addressed:
1. Without skill perks of XCOM, xenonauts 2 needs a lot more weapon/tools to keep things fresh - none of that lazy +1 variety of the last game please;
2. Rework the pacing and flow of the latter part of the game so things wouldn't grind to a halt because you have to deal with 3 battleship crash sites every wave;
3. Better ending! I cannot stress how much that phone-in ending of the last game prevented me from starting a new game after I finished it the first time.
4. Mission variety. I don't mean more maps; I want mission goals like recapture army base, assassination, escort, capture Aliens etc. Crash sites gets boring after your 50th UFO clean up.
A purely cosmetic update wouldn't be enough since X1 has its share of problems - problems that are more pronounced after various mods over the years. As the devs for the Banner Saga learned, you can't count on customers who purchased the first game to automatically purchase the sequel (their sequel only sold 1/4 of the original).