Yeah, Mario Badminton!
MARIO: I'm-a gonna make you see birdies!
(smashes birdie into opponent)
Ok, evil idea. But Mario Badminton would be interesting to say the least (it's more original an idea then volleyball).
As for Kamek's comment on never seeing a volleyball game, there has been a volleyball game on the SNES before. Except that it featured nothing but tall female humans and bad play control. Talk about the game you don't want to be caught playing.
Although I never ended up getting a Virtual Boy (it didn't last long anyway, but I did get to play a little of Red Alarm), I remember the preview in NP about Mario Tennis, and how the characters were in uniforms.
I just HAD to throw away that issue by mistake, the one with the purple cover, our first look at SMW2: Yoshi's Island, and those 3D glasses (plus the ad of the boy looking inside a beehive of some sort).
There is one idea that we haven't considered though: Mario's Halloween Hopskotch! (Possibly the worst idea for a sports game ever)