Probably the best fighting game on NG
If you ever add any more characters, make a sequel (and I'm sure you've already had suggestions, or thought through possible chars already) some good chars to add could be -- Housemaster, Uberkid, the Dad (from Dad's home), Dad n Me chars, maybe a badger and/or snake, Bitey, Colin Mockrey, Gary Brolsma, Jerry Jackson, I dunno
Not sure where to improve on this game -- the challenges are fun and give this game great replay value, the attacks and combos aren't overly complicated. This game is almost as good as it gets!
Possibly add small adventure areas (similar to Smash Bros.--destroy the targets) to open up more unlockable chars/items/levels - etc.