A lot of times in real life, there's just not one winner. And we often try to make deals and find ways for everybody to win. It's not always possible, of course - sometimes you simply want opposing things, like how the Coalition wants people to support the government, while the Taliban wants to oppose them.
Inspired by New Angeles, amongst other game, here's a variant win condition for COIN in general, although it will probably unbalance the current games - but maybe for a later entry in the series?
Simply enough, if two sides pass a victory check simultaneously, they both win (the rest lose). If three or four sides pass a check in the same victory phase, they all win - no tiebreaker. If nobody passes a victory check on the last prop card, everybody loses - the conflict continues with no clear end in sight and everybody is miserable.
Applying this to some of the old COIN games we see that it can drastically change the dynamic in interesting ways, but the games aren't really balanced for it. In Andean Abyss the government and the FARC will still be deadly enemies fighting over the hearts and minds of the population, but the AUC sails up as somebody who can play both sides - they could trade Terror in support areas for a FARC promise to not build too many bases, or they could ally with the govt for a deadly attack on FARC without worrying about getting stabbed in the back to appease the US. The Cartels probably break this dynamic hard, with no victory condition opposed by the others, they are likely to simply quietly amass wealth and build drug labs uncontested.
Adding this to a new COIN allows more solid alliances, but careful selection of win conditions and ways to achieve it should still be able to generate plenty of juicy conflict and hard choices. But you'll never have to backstab your ally just because they're doing too well.