I would have at one point too. But Minsc was datamined as one IIRC, and he was dropped as one and turned into a conventional companion, and Helia seems to have sorta quietly faded away. So Karlach, probably? IDK why we haven't even gotten a hint of further playable characters so close to release though. Unless it's some sort of huge hype bombshell they are holding till the last minute to drop before release, why keep all of that under wraps? It would have made much more sense if Minsc was still slotted to be a Origins, but he isn't, and they already announced him, and he was probably the returning party member with the biggest name recognition clout in regards to D&D since WOTC has turned him into something of a mini-mascot.

So while I expect Karlach, IDK about anyone else. And Karlach surely isn't *that* big a deal that she's being saved as a lastminute hype-builder.