I don't usually suggest anything here, but I did have this idea, which may or may not have been done before:
Nymphomania:
All decks must include at least 6 copies of a card with the word 'nymph' in the name. Thus, anyone can play as long as they have six nymph's tears, but you can also substitute and/or add any nymphs you have. Alternately, it could be -exactly- six copies of a card with the word nymph in the name, to keep the playing field a little more even between veterans with a dozen nymphs or more, and newbies who might only have one or two.
All other creatures are banned. All shards/upgrades are banned. Certain 'asshole' cards are banned; discord and earthquake for sure, since they'd wreck the theme really bad.
All decks must be exactly 30 cards. (or some other fixed number)
Eternity and Reverse time are banned. (basically, to prevent any deckout wins except by cointoss)
Not sure whether CC, shields, weapons, and other types of damage should be allowed or not; I'd leave that up to the TO.
To make it really weird, it could also be required to make trios or even quartets or something like that, specifics left to the TO but maybe minimum 6 cards per element or something.