Too small. I want a sand king XL and I want them to enable some custom zombie apocalypse stuff in the customs shop. I want these in GTA:
For the back of the Sandking/Yota trucks:
Make it available once you've won X amount of races with the truck, completed X survival missions, X deathmatches, etc. Mods that require chains of things to unlock, but make your vehicle seriously badass once you do.
For the Suburban and FJ and Hummer SUVs:
I want to see 8 on 8 vehicular combat similar to BF4 but in the world of GTA. Even 4 on 4 would be fun.
I want Cat and Mouse races. Game mode me and a bunch of friends used to play a lot on Project Gotham. You take a mini cooper/compact car and a super car and that's your team of 2. Goal is to get your team's mini across the finish line first. Super car is to ram your own car/push it for speed boost, but also to take out enemy team minis. Their super car will be coming for you and your mini too though. Usually played with 8 people, 4 teams of 2, around a longer track like the 'ring. Frantic, fast paced racing with plenty of lead changes.
For that matter, I'd like to see them enable a racing mode where you can mix and match vehicle classes in general and still race. There are some sports and coupes that are damn near if not just as fast as some of the slower supercars. Around some of the more twisty race courses, the supercars don't have nearly as much of an advantage.
I've been enjoying GTA racing too - use a class other than supercars for the most fun. I wish you could partner up for GTA racing (not sure if you can actually, haven't tried). Imagine the possibilities though. Transitional races would be fun too - a race that starts in a car, moves to a dirt bike, then a flight, and parachute in for the finish. You'd have access to all vehicles right off the bat, but make checkpoints so close together and on the ground that a helicopter would be impractical to get in right away. Include going through tunnels/under bridges/through tight alleys and stairs to discourage cars or choppers on the motorcycle portion.