A pikmin rp can work if done right, and the only way I can think of to make it work involves it not actually being an RP.
A team of all pikmin (no leader to sit around, do nothing, and tell everyone else what to do) who work together to complete goals or fight another team of pikmin or something.
A pikmin rp would probably end up with people just hanging around the onion and going out every so often to do nothing productive.
Hell, I bet I could do a project like that on my own with events. Maybe I'll give it a shot