How abouts an "out of your comfort zone" competition.
As in:
Those that write acoustic ditties instead write an electro piece, or a heavy rock number.
Those that write electro dance instead grab a guitar or piano...
Etc etc
Anything can be entered so long as it isn't from your normal style(s).
Might be a little tricky to enforce but it could be (a) interesting and (b) a good exercise for people.
I suppose it might be more demanding, which could be a pain, as you'd need a decent recording programme and/or access to instruments other than your usual axe.
Just an idea tho!