I dunno if I'm getting ahead of myself here, making engine level suggestions this early in my membership here, but I had a thought.
Grenades, or explosions having a shotgun spread in 360 degrees with a definable number of shots, and damage for those shots. Grenades in XCOM are great, but I've been a bit spoiled on games like S.T.A.L.K.E.R. where they actually send out shrapnel for damage separate from the explosion itself. I'm also curious as to whether there might ever be a split of the grenade priming state, and the proximity grenade priming state so that both can be placed on the same grenade. Maybe so that the alien grenade has an early advantage for being used early. I dunno exactly what you guys can add though, since I'm more used to zdoom. They have fairly frequent code extensions and I don't know that that's really your focus.
I guess to clarify since I get a bit rambly: Some ability to define a sub-projectile, a count, and a few other extra variables for explosive projectiles or weapons. This could allow for cluster weapons, grenades with low damage AP shrapnel, alien grenades with plasma bolts, and other more complicated sub-munition behavior for weapons. I will freely admit I don't know the engine limitations that would cause trouble with this.