Time is so flexible in Star Trek, anyone could turn up.
Kirk: An echo of him was still in the Nexus, which can now be entered at will.
Janeway: Voyager passed through some sort of temporal eddy on its trip home. Upon their return, she swore her crew to secrecy about the future.
Sisko: Leaves the Celestial Temple for his next task.
Archer: Drops by with Daniels discussing temporal cold war matters.
Picard:
Kirk: An echo of him was still in the Nexus, which can now be entered at will.
Janeway: Voyager passed through some sort of temporal eddy on its trip home. Upon their return, she swore her crew to secrecy about the future.
Sisko: Leaves the Celestial Temple for his next task.
Archer: Drops by with Daniels discussing temporal cold war matters.
Picard:
Immortal synth body