I am looking at it from Matalas’ perspective and what he wants to do.
There’s no point for a mass Voyager reunion film at this stage if they are ever going to do one. Too much uncertainty. But a couple of cameos could happen in a Legacy film. VOY cameos aren’t without precedent in Star Trek films.
DS9 is also unlikely. The best that could be hoped for is a Kira cameo and maybe a cameo of the station and Bajor.
I also don't think a Wesley appearance with Robin Lefler would be impossible. It would be another way to erase the bad taste of NEM.
The PIC writers chose to give Picard a son instead of building his relationship with Elnor. And it hard to ignore some sort of meeting between Elnor, Jack and Jason Vigo occurring, even though it should be left to television. And they also gave Picard a synth body that he has never really bothered to discuss.
While I can see the transwarp conduit being left for a later show (such as something based around the Ent-J a century later) and would even be content with it being the case, the PIC writers did say that it was being saved for later. That implies Legacy, though there is no rule that it has to be answered in Legacy. Frankly, I don’t care if the transwarp conduit is followed up on anytime soon. Not do I really care about the synth alliance. Not really important. I’d soon see a red wormhole appear over DS9 (readers of the DS9 Millennium novels will understand that reference). But I would understand why the PIC writers would focus on what they created.
The PIC writers also created the Confederation timeline and the pandemic screwed everything up. So its understandable if there are those that would be interested if Picard met General Riker.
With Patrick Stewart and Levar Burton pitching a final hurrah, it should be discussed what it would be like if Paramount went through with it.
A possible idea would be here. Its lowstakes and I think a lot of fans would like a lowstakes plot as well, as would Paramount because it would be mid-budget. Star Trek: The Big Goodbye does sounds right for a final TNG film. But let’s say Paramount need stakes and think reflecting on memories over a poker game is too low stakes for them. And a Tarantino film still isn’t getting a warm reception from the fanbase. There are better ideas out there. In which case, dig into the TNG well to find something that would fit TNG.
I do admit that if Armus was the antagonist, it would give Troi something to do and utilize her counseling skills and avoid the mind rape trope. And with the bluegills & “Conspiracy” parasites being the antagonists, it would give Beverly something to do. And with either antagonists, the final act being Picard and Riker doing the Star Trek version of the film Logan would work. that sound fresh and like a TNG film. It would just be a matter of figuring out how Data, Geordi, the La Forge daughters and Data’s children on Coppelius could be worked into the film. And working Seven of Nine into the film.