I've noticed a lot of people saying that they've finished 2.2.1 -- they reach the point where there's nothing new to research, and the battles are all easy.
Since 2.3 seems to be a while off, how about a simple quick-n-dirty patch for 2.2.2 -- increase the difficulty with time. Yes, you get more powerful, but the aliens do also. National funding decreases to zero, so you have to rely on UFO parts and manufacturing. Eventually you'll actually go broke or get wiped out.