I think modelling will mostly get the most entries as it's the one part of 3d pretty much everyone will have tried out.
Even if someone specializes in one field which is different, say animation, texturing or vfx, they will have had a go at modelling at some point, and a lot probably do it in the spare time for fun even though their job doesn't require it.

Another idea for the challenges would be along the lines of just a subject matter, as some seem to have felt the guidelines are quite specific, so maybe just use key words, like some other site's comps do, and I think I've seen that done here too. The one Arran won a while back? Decay or destruction or something?

Just that way it gives you a theme to stick to, but a great deal of flexibility and freedom with what you do with it.

Maybe even have two key words that are quite different from each other, and see what people make from that, like "Trapped" and "Slick" or "vast" and "Chained".

Could end up with some interesting results.

My two cents, pence, whatever....