Yeah, i did think about how hard it would be to implement, but i don't think it would be too hard. Fun gameplay is the most important thing for the game, and for the fun to be long lasting, online experience is what makes a game last long.
There are people suggesting hard and unnecessary things to implement like Track Editor. You suggested crazy weather events, and even a Game Mode based on it, which would be really hard to code for too!
I'd much rather see time and effort go into a diverse range of race tracks and environments, yes, but Arena mode should be next on the list. I think if 'Grip Soccer' could be made to be really fun online, then i would see it as a reasonable stretch goal. At least, i put it far before things like Track Editor. We don't need that IMO.
I do think 'Grip Soccer', and more importantly 'Arena mode', would be something which makes the game a number of times more fun and longer lasting in the online community, and thus very important modes to think about implementing and putting forward above other possible 'stretch goals'.
Combat against your mates online? Yes please. Who wouldn't turn that down. Especially if there is tournaments or league tables in itself. It could make people addicted to the game more than track mode itself!