Nimoy's Spock might be the only 24th Century character that appears but that doesn't mean that others, such as Captain Riker, won't be mentioned in dialogue. That's certainly the next best thing to them appearing, a nice nod to the fans. I doubt that any major 24th Century character will appear. Maybe a lesser known one, or perhaps a completely new character/characters.
Spes aspera levat - Hope lightens difficulties.