hmm very hard question. I feel like the main "plot block" to solve is how the two worlds shown in the trailers interact with each other. As we have seen Scarlet is connected to an orange and we have clothes of the same color and that's the same thing for Violet with the purple clothes. So, this might be connected to the Legendaries. In Scarlet, the "dragon of past" is venerated, while in Violet the one of the future is. The Professors are different: the one in Scarlet wears clothes giving the idea of past, while the one in Violet wears a cyber suit. With no much surprise and as everyone said, it really is past and future. However, will the story actually be "past vs future"? I mean, do in Scarlet games exist people who are rebels of the "veneration of the past" and in Violet the opposite? That's hypothesis one: Evil Team wants innovation in a game where traditions reign (and vice versa in Violet). The consequence is they awaken the Legendary of the other game version and we have to team up with the Legendary of our version and defeat the Evil Team. Like in Ruby and Sapphire, but in the opposite way. Honestly? Imo it's too basic and also, there's a huge detail: the two pokemon in the trailers look in different directions and seem somewhat not connected to each other. This could be a sign that this time the two legendaries might be unique (in Scarlet universe it doesn't exist the Violet Legendary) and they are kinda similar. But, that's impossible because otherwise you wouldn't be able to get the other version legendary, because plot wouldn't justify it.
I have a crazy hypothesis: They're actually the same pokemon that evolved or had a different development in the two game versions stories. I think that might be too crazy tho lol.
However there are other paths, especially if we consider two important elements: the third legendary and the mechanic. Swo/Shi were really clever about this: the plot bond together box Legendaries, the third Legendary and the mechanic. So, it's right to take into account that this could happen again. We have no clue on the mechanic, but we know that it has to do with a star and with light, so maybe it's connected to pokemon types. Because after all, Violet and Scarlet aren't connected to the idea of "future", but to light spectrum. So, second hypothesis: again we'll see a plot about the Legendary being connected to the game mechanic and the third Legendary might be involved in that.
I am very keen to think that this time we could get no third legendary, but it's too improbable, so yeah just forget about it lol.
About the rival, you might think that she could be the evil one, that has different believes than us, like N basically. However, I don't think so because in the trailer she is shown wearing the uniform of our same color and she looks really friendly. Considering the role rivals had in the last games, I pretty much doubt she is going to have the development of the rival that is friendly at first, but that then wants to stop us because she has different ideals. I don't think it's going to happen in a pokemon game. Those are mainly "theories".