I want a mod that let's you continue running X-COM with volunteers after the Counsel tries to shut you down. The CFN becomes your enemy and send Navy Seals, SOCOM, legions of the best from around the world every month.
That's not what I'm talking about. This next section involves changing a lot of how the game rewards you. So bare with me. These are rough ideas that may or may not be possible to implement.
If you need a challenge, this is it. You won't worry about the Counsel's point system anymore, but funding and supplies will have to come from a 3rd party, maybe corporations and strange factions you can accept or deny. You will be juggling these groups and some will pay well, others not, though they'll come with different benefits and perks. You'll also need to worry when a group nags about you accepting funding and benefits from their enemy or if they become enemies with a current funding faction member. Anyways, benefits are as the original when the Counsel did everything for you, but you won't get everything at once. Benefits come from giving you a recruiting roster to hire new operatives, to being able to safely supply you with certain arms, armor, accessories. Perks would be quicker injury recoveries, to cheaper building costs, you know the generic gaming stuff. Also if a dictator takes on to your cause, you'll be able to build bases without any retaliation. Building anywhere else gives you a small chance of getting discovered by that nation and the Counsel finding out. You'll be building bases in a more secretive manner, so you'll be okay. It's just the known bases, before the CFN disowned you, that you'll have to defend.
I could go on, and I did write a looong topic about this awhile back in the suggestions. You can try to look for it if it interests you. Others chimed in as well.
Edit: I found my old topic.
https://openxcom.org/forum/index.php/topic,1542.msg14101.html#msg14101