There are plenty of sports titles out there that more accurately represent that respective sport, but there's only one Mario, which takes the basic concept and runs with it. I don't want that to change. Mario Golf is actually my least favourite of the Mario sports because it is too much like the real thing, there aren't enough extra modes that provide spins on the formula or fantastical environments. Mario Tennis has this problem as well, but tennis is more enjoyable as a video game. Mario Power Tennis had the right idea with the special courts, power shots, and minigames, though it's missing a little something I can't quite nail down... It also has less characters than the Nintendo 64 game which is rather odd.
To me, that's what the perfect Mario game would provide. It would have the basic sport for those who want it with the personality of Mario characters, but it would also have a lot of modes and features that center around the concept of the sport and mix things up.