I mean, in VS racing you can change the rules before the races start and obviously you can set up custom race rooms online with access to the same rules selection
While it's entirely possible for Nintendo to be boneheaded enough to not allow the same freedom -- making it so once you select the mode you go straight to the character select screen -- it would be really strange because I'm pretty sure I remember at least previous MK battle modes (Wii mainly) allowing you to at least change the time limit.
So, assuming Nintendo isn't stupid the mode itself would be Balloon Battle but you would be taken to a rules selection screen where you could turn off CPUs, set a time limit (or not), set score or set it to Elimination/Survival, and set the item type (just like you can in VS races).
In fact, them NOT offering any options would fly completely in the face of how much they advertised the options you had for racing. Remember how much they talked about how you could change the items and so on for races? It would be really, really weird if the same options were not available for battle modes.
It's much more likely the main matchmaking for battle mode will just randomly choose one of the five modes.
However, that wouldn't mean you wouldn't be able to set up your own battle room like you can your own race room and set the mode you want it to be for people to find.
edit: hold on i'm a dumbass
Most of those options actually are there already in the Wii U battle mode, it's just pretty limited.
Round Time for instance doesn't have an option to disable the time limit, just from 1-5 minutes. Of course they're not limited to these options in Deluxe, but there's precedent for not letting you customize much.
This has me so conflicted... With options it could be a fantastic new mode that is even more substantial for me than the original racing modes. Without or too little options I might not even buy the game again.