How about a word limit? A lot of interesting cards are those with walls of texts describing there effects (guilty). But you can still do an awful lot while being brief.
The contest would start with a set limit of words that can appear in the effect boxes of the card (that is, Name, Dex, and Rule boxes/bars do not count, but abilities, attacks, powers/bodies, and special rules like the energy rule on
Holon's Castform would). Contestants would be allowed to exceed this limit, but at a penalty (perhaps 1 point for every 2-4 worlds, rounded up?).
The point of such a contest would
not be to restrict creativity, but to promote it by way of an alternate thought process. You can no longer simply come up with some convoluted idea and then refine it, you must instead start with an empty slate and fill it up with as the most interesting mechanic that fits.
If this were actually implemented, I would recommend a limit of around 20-30, but that is at the discretion of the judges.