I'd love to see a full-on custom campaign project, personally. If I had the concentration and such to learn how to code, and do all this stuff, I would have already started. Whether these get used or not, a custom campaign will be great regardless of what it is. There are a few custom campaign ideas I have. Play as Advent Basically, swapping roles in the campaign. This will be the hardest task of any other custom campaign, I would think. Avatar Project logo on the map would be Advents commanding base, and exterminating XCOM would mean victory. (A typical) Zombie Apocalypse campaign. Because there are totally not enough zombie games out there. Have there be no guns on their side, no UFO's, no research. Instead of avatar project, have an infected meter which only increases if you can not complete an important mission. (Like rescuing civilians from a zombie raid) incredibly slow research for XCOM Reskin the avenger to a vehicle, and prevent flying cutscenes when base comes under assault. (Instantly defensive state, survive the oncoming waves) (Maybe I'm asking for the impossible) Co-op campaign This one speaks for itself. Instead of 2 XCOM bases, or sharing the same base and troops, One player is XCOM, the other is the resistance. The resistance control only the areas inside the avengers contact area, searching and spending time looking for areas of interest for the XCOM player. Actually attacking ADVENT convoys to take them out, appearing on the map after a successful raid, allowing for XCOM to take over afterwards. You get the general idea.
