I doubt they'll go back to 3&4 after P5 releases and at this point they're really pushing it with extending the story anyway.
The thing is that it isn't that simple. They've put a ton of work into creating characters, stages, music, art, etc. for them to simply throw it away in a potential P4A3. Roster choices are obviously relevant to the story, though, so would that mean the whole cast would be returning for some reason in a sequel? If not, would they really throw away that work, forsaking the total number of content and the fans who already got used to them before (e.g. playing certain characters that would be removed)?
They can't just refresh a P4A3 that would have a total number of 12 characters that were either from P5 or original; that would be senseless. But then would they try to shoehorn another storyline involving P3 and P4 when this is supposed to be the grand conclusion to the P-1 Grand Prix? There are a lot of questions like that about the possibility of a P4A3, like would ASW and Atlus continue to go the route of using sprites, or would they use Xrd type models? Maybe, relating to the latter, we wouldn't see a P4A3 for a real long time (like 4-5+ years), only for it to return as this refresh, which would make it more sensible as it wouldn't be so closely tied to the first two games.
All we know is this: P4A is one of the most successful modern fighting games in terms of reception and sales, and that the game's director has stated the following:
Wada: Persona 4 Arena" was our first attempt at a "Persona" fighting game, so we focused heavily on making sure we had a solid foundation with all of the basic components a player would expect from a fighting game. This of course means that I at least want to see how far we can expand this horizon as well. There are lots of new characters I want to bring in, and I would love it if the name "Persona" eventually earned its place as a staple of the fighting game genre.
Marie and Margret aren't going to have story DLC. The P3MC dream is dead.
They're still most likely going to appear in the story itself, though.