@Secret_Tunnel Yeah they really haven't talked about it in detail from what I've seen...there's just a brief blurb on the website about it that doesn't really explain anything:
Marble It Up: Mayhem! said:
Embrace the intuitive and chaotic multiplayer modes. Hunt gems, tag your friends, and prove your marble mettle online. Blast is back! Gem Hunt returns! Steal the Crown debuts!
And the only video online I can find is this, which I'm guessing is the Gem Hunt mode:
They did mention in a long pre-release preview video that they're working on a mode inspired by Battle Royale games, but from quick search in the Discord it looks like that's not ready yet.
As for the singleplayer pack, another Youtube video I found looks like the Mayhem game on Apple Arcade has five chapters, and the levels in the first chapter look like about 70% of them are new. So that's probably a pretty hefty level pack for the "base" game on Switch and Steam.
No word on timing yet either, but they're doing some multiplayer beta weekends on Steam starting this weekend, so I'm thinking maybe we'll see the update on Switch in the first few months of 2020?