Cards that untap lands on opponents' turns
You can do a quick search for keywords like 'untap' + 'other player' to find some cards.

This'll put you on Seedborn Muse. She's the original Prophet of Kruphix, but hasn't been hit by a ban because she doesn't have that super-juicy "flash" clause. (And because being mono-green means she props up more diverse archetypes than a UG card does.)

Is that your only option? Not quite: try other keywords (maybe filter down to your colors and search 'untap' + 'land' or 'untap' + 'permanent').

Some highlights that you're likely to miss unless you try a lot of search permutations:


Awakening is a green enchantment that untaps everyone's stuff. (And allows heavily-instant-speed decks to double-dip mana on their own turn.)
Patron of the Orochi is a big green creature that you can use to untap all your green creatures and forests each turn. (And can also be used to double up your mana on your own turn.)