Quick moderator note: Any comment about how TDR is horrible will be deleted or edited. This is not what the thread's about.
I know, I know, we don't talk about TDR. But here's the thing: I think there genuinely were some good ideas in it, and I believe some of them could be adopted for C:R. Here's styg's list of Five Things From TDR2000 I'd Like To See In Reincarnation.
Peds who fight back
This would be my main argument against zombies, I think. Zombies just kinda stand there and take it, or shuffle out of the way. Human pedestrians can kick your windscreen in, attack you with weapons and explosives, or even push things off buildings or cliffs onto your car as you drive past. Now, TDR's implementation of this was crap and the peds never really damaged you, but the idea was there, at least.
Cops shown on map
I like hunting down cops in C1. I don't like the fact they run away as soon as you set them on fire. I especially don't like, if you've lost track of them, having to drive away from the area to let them reset to their original location. With cops showing up on the map, as in TDR, duelling them becomes more about the duel itself and less a matter of waiting for the system to catch up with itself.
The verticality of the maps
C1 and C2 were quite good at this, in fairness, but TDR was best at it: Maps which felt truly three dimensional with rooftop trails, flyovers and proper sewer networks. I love that sort of vertical gameplay, especially if you can take the short way down to the level below. Devil's Canyon mines might be my favourite C1 map for that reason. Ohnhai's teasers of the new Industrial Injury make me happy because they show off much more attention to the level as a 3D space than the original map did.
Free play mode
With the way C1's races were structured it was not so much a problem there, but it annoyed me in C2 that if you were in group 7 or 8 and wanted to play, say, Beaver Woods, you had to finish the game first. TDR let you go back and play any unlocked environment in free play mode, with no time limit and no advancement to be had: just lulz (and, IIRC, your APO upgrades carried over from free play to the campaign?).
I know C2 had the afterburner powerup too, but TDR's was amazing. Not just the insane amount of speed it added but the barbecue bonus, rocketing forward while in flight, the near instant switch from reverse to full speed... it's practically cheating to use afterburner, in that a solid crash using it killed pretty much anything in one shot, but I don't care. It might be the most fun powerup in any Carmageddon.