I think the main ones we know would obviously be Seven, Raffi, Jack, and Sidney. Jonathan Frakes has been loud about wanting the spinoff and wanting Riker to make appearances and for his daughter to be on the ship. I assume that'll probably happen in some form too. Other than Riker, I assume we'd get appearances from Beverly, Geordi, and Deanna given they're the actors that have said they want to do more. After that, idk. Honestly for any spinoff, I'd be way happier if the cast was mostly new characters plus Seven and Raffi. But I know that's not going to be the case if it ever happens, so I hope they resist the urge to load the ship up with every nepo kid they can find.