Simple, be like MK 9 but with the patch support of I2 and MK 11. No more variation gimmicks, ditch it, no more who can out stagger who mechanics, make wake ups good again and not shitty. Bring back breakers, get rid of the FB and breakway. Neither are as good as breakers. Oh and make Raiden actually viable and bring Ermac back along with tag mode. Story is always amazing so no complaints there and ditch Kombat League, just let us get the rewards via single player. I don't want to have to deal with the BS, cheaters and glitches just to "rank up" to get awards, skins, gear etc. Increase the speed like MK 9's dashing speed, it was perfect. Fluid, nobody felt slow yet not super fast like MK X running 50/50 either. They want rolls, again make it like MK 9 where I can roll back or forward without spending meter. I would also get rid of Fatal Blows and KB's, they're gimmicky as well. Just give us back recharge meter Super/Xray. Fix throws so that they aren't as OP, if someone ducks and pokes that should get you out of a throw. Keep 50/50's to a minimal. Make it like MK 9 but even more like oldschool MK.
This is what I would do for MK 12 but time will tell. I doubt it'll happen but my wish.