Since Prime Verse is most likely dead in the eyes of the execs, anything new relating to any of the TV series (apart from a documentary) is highly unlikely. If I was going to put money on it, I'd say the next movie, a documentary with a small amount of unreleased material and some actor interviews and a load of re-runs of classic episodes from all the series.
A feature-length TV special pulling in members from all 5 series would require cheesy time-travel or holodeck rubbish. Doctor Who can pull that off, because time travel is the whole premise of that show.
It would be possible to do a '20 years later' thing with TNG, DS9 and VOY crews, a bit like B5 'Sleeping in Light' but I doubt it would happen. Doesn't mean we cant fan'fic it though