Yeah, I love the idea of mounted combat in D&D crpgs like BG etc, but BG III clearly isn't built with the intention of that being a feature. The more I think about it, honestly I wouldn't be surprised if there was more celebrities &/or Fanservice though. Mercer's involvement +
some of the datamined stuff
wouldn't surprise me if that CritRole connection gets further pushed. Aside from that I could see more party members/cameos from the first two BG games. Minsc and Jaheria were a big hype-building reveal, so something of that magnitude being held back is a good yardstick to gauge our expectations if it's a character reveal. Since Jaheria and Minsc are heroes, perhaps one or more evil returning characters? Viconia and Edwin would be the obvious inclusions if that was the case.
Moving beyond that, what would generate more hype? IDK. Only Drizzt has the mainstream D&D appeal, but obviously Drizzt himself can't be a companion. Maybe RAS is involve in the writing of the plot somehow...? it could be a possibility, although from what I understand he's been busy dropping a book a year for some time now, so I don't know how much he could feasibly be involved. Still, we have a Baenre drow in-game, and some of the stuff in-game seems to indicate Larian at least got an early draft/memo/heads-up in regards to the upcoming drow retcons/developments that hadn't been revealed yet. So something regarding drizzt/drow/RAS could be possible.
So in summary, if it is story-related, I can see it going down several ways:
1) more celebrity voice actors. Don't know if there are many more out there super associated with D&D though.
2) something CritRole related. build on the hype they gathered by recruiting Mercer.
3) returning BG characters. Seems likely to me some way or another, even if it isn't the specific thing Sven is excited about, no way only Minsc and Jaheria are the only returning cast members.
4) something ras/drizzt related? RAS has collaborated on and written for video games before, and BG I + II had Drizzt cameos. Some aspects of BG III seem to have been written with the drow changes in mind.
Could be other possibilities I'm not accounting for-it is Larian after all, everything is fair game