For a runaway player, I’d borrow the same kind of controlled experimentation that makes version comparisons useful: keep the rules simple, then change one variable at a time. A short Battle Mode limit and character rotation would give newcomers room to settle in without turning the session into a handicap tournament. I’d also make the warm-up non-ranked, since Super Mario Kart is much easier to enjoy when learning a corner or item pattern doesn’t immediately count against you.

I like the “one variable at a time” idea a lot. For a short couch session, I’d keep the non-ranked warm-up, then rotate characters every few Battle Mode rounds so nobody gets too locked into one comfort pick. A small first-to-three balloon limit also sounds replayable without letting one hot streak take over the whole night.