Why do you think it will be story related? Is Marie's inclusion story related? I had assumed she wouldn't have a story mode.
We have no information concerning whether or not Marie is related to the story, however this has been the modus operandi for literally every character who's part of the roster currently, for Atlus' motivations behind character choices for the game, and for ASW's character choices behind their own fighting games. I currently see no reason to believe otherwise.
Where do you find this? If it involves scans, then I'll look for it myself.
For the next character reveal, I'm betting on Makoto or Fuuka.
The signs were ones that had been present before the reveal, and were made relevant after it. I had not thought much of them from my natural scepticism, but now they seem far more important to me. Stuff like:
1. Yu Namba's quote after the P4AU exhibition. He said something like: "We already have one DLC character announced... more to come, hey."
2. Special DLC cards. There are going to be 37 of them, and we know [Redacted] is number 035. It is very probable that Marie will be #036.
3. Character screen layout. Not as relevant as the other points, but they'll have to maintain symmetry unless they want it to look like a mess, and in its current design that would require two more additions to the current one instead of one unless they were to completely revamp it. There's also the part where I really think P3 and P4 should have their own rows completely, but that's a lesser point.
4. Every month since April has had its own character reveal. April, May and June, didn't mean
too much, but July added onto that is too much. This is combined with the fact that, if Marie was the last character to be revealed, why wouldn't she have been revealed in August as a last promotional boost since she's going to be post-launch DLC?
5. Intuition. I had displayed my confusion previously as to why [Redacted] was revealed so early if he were to be the last character several times. More than two months before the game launch seemed like too much if they were to maximize hype factor when it's something that could have been kept for last. I am now asking myself the same kind of questions when it comes to Marie: why would they not have switched the Marie reveal with [Redacted]'s if she was to be the last? It could be related to P4GA, but I would think the former would be more popular and currently has openly demonstrated ties to the narrative, so it just seems like they would have naturally been the last one instead of Marie.
A lot of that were things I had glossed over before because I doubted any character reveal after June's, but now I'm optimistic on the prospect.