Oversoul
The Tentacled One
So if you like infinite Regrowth loops as much as I do, Bonus Round sure can help generate them. It'd take quite a bit of mana to get going, though, and I'm not convinced it's even a particularly strong use for the new card. We'll have to wait and see. But it was something I thought of early on. For example...
1. Demonic Tutor for Bonus Round.
2. Cast Bonus Round.
3. Cast Regrowth, it gets copied. Pick Demonic Tutor and Bonus Round back up.
4. Cast Bonus Round. It gets copied. Laugh maniacally.
5. Cast Demonic Tutor. It gets copied by the first cast of Bonus Round, it gets copied by the second cast of Bonus Round, It gets copied by the copy of the first cast of Bonus Round that the second cast of Bonus Round generated. Search for another Regrowth and some more stuff too.
6. Cast Regrowth. It gets copied by all three instances of Bonus Round. Pick four cards back up, including the first Regrowth.
7. Cast Bonus Round again. It gets copied three times, which means that there are now seven instances of Bonus Round.
8 Cast Regrowth again, presumably picking up your whole graveyard.
9. More Bonus Rounds!
10. Demonic Tutor up every relevant card in your library. Cast them. Win I guess.
Where's all the mana come from? I dunno. You're going to need cards that make more mana (preferably in whatever colors your deck uses) than it costs to cast them, and you're probably going to want a lot of them. But the key point of interest, which makes this viable at least for casual play, is that once you pass some initial setup threshold, you don't really need your mana accelerant cards to be that explosive. Yeah, it'd be nice to use Black Lotus and Dark Ritual, but I could see some weird setup with Lotus Petal and Seething Song working.