I hope they add more to the game before going on a dlc rampage.
The base game is so damn fun, if only there were gameplay modifiers for local co-op and online multiplayer (non-ranked) including parties for things like ... I dunno off the top of my head:
Car size - big/small (including as a handicap for 2v1 etc)
Unlimited boost +/- vehicle kills off
Car handling/capabilities - speed, power, triple jump/single jump limit/super-jumps (first/second jump are stronger). Haven't played the rainy maps yet, hopefully they're more slippery.
Bouncy cars
Gravity changes for the cars, strength...direction? might need new maps for direction. Custom map support is probably asking too much.
Multiple balls
Obstacles, eg mines, huge moving non-scoring balls, raining heavy balls and mines
items...
Ball charges power if hit multiple times quickly (glows to indicate it, when you hit it it goes further) sort of like Mario Strikers Charged
Jello ball
Cars leak oil when they're hit/ or constantly...makes it slippery unless you're boosting, which uses up the oil
Magnets on the front of the cars that repel the ball, boost is lost, button used to toggle between repel and a much weaker attract - ball knocks you back if it manages to move long distances towards you and gain momentum
kamikaze cars (low number of hits, explodes on final hit)
...random (like dice roll) mix of some of the above mode
First to x wins instead of just time based. It might have that, haven't come across it yet though iirc.
I'm sure there's loads more fun stuff. But yeah small changes as options could make the game even funner. At least having that party/custom mode there if you wanted to mix things up a bit.
Did the first game have any of that?
I was thinking about this last night. Most of these in your list I was thinking would be great too, just private lobby or like Halo's Forge mode kinda thing. It'd be awesome fun.