The only confirmed ones are Ryker, Troi, Data, 7 of 9, and Hugh Borg. (Various old actors they used to often re-use under make up have also been cast, but who knows if they are playing someone new or old)
They did mention that they don't want everyone returning right away to give audiences something to look forward to.
I know he was also interested in directing an episode, but he's one of the main directors on NCIS so it would depend on their shooting schedule.