And of course, it'll have a gimmick that makes it slightly/very different from other games, and will directly involve scoring!
And now for some speculations that are not based on what we already have seen:
Based on patterns, it MIGHT be Phantasmagoria. Very likely.
In such a scenario, Reimu, Marisa, and Sanae are guaranteed a spot. Aya, Okuu, and Kogasa are likely possibilities. Other characters are variable.
It probably will NOT revolve around water.
If it's a Danmaku game, it will have Reimu and Marisa, with a possibility of Sanae, and if the SDM is threatened, then Sakuya will appear, too.
There will be curved lasers. BEWARE!