1.) Patrick Steward as a retired Picard, who appears in the last five minutes of the pilot episode and gives a short, powerfull speech in honour of the maiden voyage of the new ship and crew, shakes hands with the new Captain, and then leaves to his wine fields.
Then: NO cameos for the first 2 seasons of the new show! Let them find themselves! After that:
2.) Jeri Ryan as Annika Hanson . After the (inevitable) first Borg episode, Starfleet seeks advice from the former Borg on the new kind of Drones our new crew has encountered. Annika is less than thrilled, and had hoped to finally having left this part of her life behind her, but is now exposed to her past and conflicted feelings towards the Borg (and humans!)
3.) William Shatner. A bunch of Orion space pirates have gained acces to a DNA-probe of the legendary Captain Kirk and have cloned him in hopes of gaining iside from his legendary intellect and tactical experience. Instead, Nu/Old/EvilKirk has taken over the whole pirave hierarchy. Let Shatner ham up the scenery!
4.) Kate Mulgrew. As a grumpy, cynical Admiral Janeway, that orders our new crew on a questionable mission. The tension rises, as our Captain wants do the morally right choice, but is ordered to to the questionable thing that benefits the Federation.
5.) Jake Sisko, as a valiant journalists on the frontiers of known space, whose endearing writings focuses on the needs of the socially deprived, who are forgotten in order to benefit the people at home.