id software are the godfathers of FPS, I think they could pull off the basic Doom gameplay without needing ton of different mechanics. Focusuing on superb sound and shooting/gun design, add something like slight variety to different base enemy classes (spikes, horns, color, weapons, outfits etc).
I also think we are long overdue for proper procedural dismemberment system (the E3 2015 Doom gameplay had some interesting things happen, like half-torso blown off, legs blown off with upper half crawling) and persistent corpses (this was also in the E3 2015 demo).