Got more ideas for challenges:

Team battle. Each character can (for the duration of the challenge) summon up to two partners, underlings, sidekicks, what have you, from their respective universe, to then compete against the other challengers in some manner or another. Just a chance to expand on the interactions of each of our respective settings.

Planechase duels. Another possible dash of the Magic setting, each challenger is temporarily granted Planeswalker-esque summoning powers, to then compete in Planeswalker duels (aka Gatherings) across various rapidly changing battlefields.