Maybe Brent Spiner will show up as Data or Patrick Stewart will appear as Picard (yeah, I know they say they made it where he'll have a "natural" life-span in his synthetic body, but just like with Data's decision to end his life, things can change). Although, it might reek of being similar to Enterprise's finale, where characters from other shows play significant roles in Discovery's finale. So I would understand why they might want to avoid that.
I don't think it's Discovery's place to reveal what ultimately happened to Picard. That should be left to something that's either Picard or TNG-Related, like the Picard Movie or Legacy.