MewLeDorf

Bit late to the topic, but the main thing would be the race/speed/timetrial events as they were in TDU 1, just races scattered across the map rather than all being tied in a bunch of championships. Hated that in TDU2. And make them repeatable for the same cash reward. In TDU 1 I did the island time  trial for 1 million to get money, in TDU2 I had to do a ton of casino to get the stuff I wanted.
It'd be fun if you could tune the looks of cars like in NFS, and to some extend FH.
The return of motorbikes. For some reason I like discovering roads better on motorbikes. Also, if there's off-road again, dirt bikes would be neat.
Allow people to use DLC offline. It's stupid I paid good money for the casino DLC and the motorbike DLC and I couldnt use either after the servers went down. 
If it has storydriven content, please make it have a better story than TDU2 which was simply horrible. 
I liked the way in TDU you could do the white icons driving people around for clothing and the races, delivering packages and cars for money. It worked. I feel TDU2 emphasized the fashion aspects a little too much, and I don't care for it. (especially annoying in FH4, hope they steer clear of that way)
Surely can think of more but this is enuff for now....