1) Foes that be somewhat evenly matched and ideally, share a power or ability that is similar in nature (think along the lines of 'Hawkeye vs Green Arrow').
2) Opponents whose powers are opposite innature (e.g. 'Iceman vs. Polar Boy').
Additionally, while these match-ups do not have to be strictly 'Marvel vs. Dc', the pairings should be novel ('Thor vs. Loki' would not qualify).
As far as which version of the character we are considering in the contest: pre-crisis, post-crisis, the multiverse, etc., let's make it the character who is currently featured, and residing on the same Earth as you an' me.
Contestants who have died or effectivly died due to their series being cancelled are also eligible. After all, dying is often just a minor inconvenience in comics.