I think gameplay-wise, things'll probably start over from scratch again, like in the Xenosaga series, though maybe with a bonus for importing save content from Part 1. I'm not sure how far Part 2 will go in the story, but I'm guessing it may be very different than some people think. "The unknown journey" and all that. It may diverge from the OG more than many of us are expecting. SE's already shown us with Part 1 that they're willing to change things up, even significantly if need be.
And I'm not so sure that the playable cast roster will be exempt from such changes when virtually every other part of the game has had changes made to it. Adding a new face - just one, mind you, since it's already a sizable cast - would be an interesting way to mix things up, subvert players' expectations, and keep things fresh as well as give a new perspective on a once-familiar journey. Assuming she survived, Jessie would be the most likely candidate for such an addition. She's already been given many things that are often only given to playable characters - a well-developed backstory, her own musical theme with multiple variations, a connection to the main protagonist, ties to places along the journey, and reasons to make that journey in the first place (atonement for what she feels she did with her bombs as well as finding a way to either save her dad or avenge her parents, depending on whether they're alive or dead). She could also be a way for SE and us to defy the tired old trope of "childhood friends must become lovers" that Tifa represents so we can still have a choice of who he ends up with after Aerith has her inevitable date with the Masamune.
I'll freely admit it's a long shot, but I don't think it's as impossible as some might believe. Especially with how her popularity's been shooting up since the remake came out. Much like how Zack become really loved and popular despite having only a small part in the OG, popular enough to get his own game. Jessie might end up following a similar path. Not her own game, necessarily, but a larger role in Part 2 and beyond is certainly possible, I think. Her connections to Avalanche command and her history with the Gold Saucer and the play that's central to the date event make it a good bet that she still has a part to play in the story one way or the other. But letting her join the playable cast would give us the opportunity to see growth and interactions with the rest of the crew that were never possible before but which could be fun and interesting to watch.
We'll just have to wait and find out what SE's going to do, I guess. But with all that she's been given in Part 1 and how she's been developed and received, I think she's got a good foundation if they do go that route. Almost like they were prepping her for it, since it wouldn't have worked had they tried it for Part 1 when not many knew about her yet or what to expect. But now, things are different. Her popularity in the fanbase is high and still rising, she's got motivations to make a journey away from Midgar and the connections to help her and the others along the way, and a combat style that could be expanded if she were playable - imagine her fighting with two guns Dante-style instead of just one. That would set her apart from Vincent, who typically only uses one. Or having a combination style where she can mix up some martial arts moves with her gunshots (she does have some knowledge of hand-to-hand fighting as we saw when she kicked the soldier on the train platform). Combos like Tifa, but utilizing both kinds of attacks.
Just speculation, of course, but I think it could be possible, even if it's still a long shot.
Cloud should have a nice thin scar on his chest/shoulder somewhere and Tifa… Well I don’t know what they’re going to do with the scene where Sephiroth slashes Tifa, he will probably just push her really hard down the stairs or something. Judging from her blue dress she definitely doesn’t seem to have a scar between her breasts as I think was a somewhat popular headcanon in the fandom, though I guess she might have one around her midriff area.
