I'm really excited for this game. StB and DS are great games and I expect this to be no different.
Looking back at those two games, we can also assume a few things - like the cast of characters. Both of these games included almost all characters that were newly introduced since the last game of this sort, so ISC will most likely have all characters from TD (Yuyuko), DDC (most confirmed already) and Kokoro. As there is one main game less than usual, I expect older characters to mix up the selection (Keine). She is fitting to team up in a stage with Kagerou. Prismrivers would fit the Tsukumos/Raiko (+ they never where in a StB-like game before). We can also expect all Players from TD/DDC so Reimu, Marisa, Sakuya and Youmu (most likely with Yuyuko?).
Other likely possibilities for me are other Tsukomogamis like Medicine or Kogasa (she was also in TD) and probably a new Final Boss like Hatate, as speculated by Seija herself.
Also Cirno as she also reappeared in DDC, i guess?
I also assume the Items Seija uses aren't necessarily stolen or realted to characters we know (while obviously inspired at points, like the umbrella and ying-yang orb) but instead are items similar to the cursed weapons in DDC. Seija probably gathered all or at least some of the other items in Gensokyo, that were affected by the Magic of the Mallet and is now using them to protect herself/fight back against the hatred of all of Gensokyo.
This also makes me think if we are entering into some sort of "Tool Arc", with some items like these falling in the wrong hands and causing incidents for the next few games, thought that's completely up in the air.
Totally looking forward to next month!
PS: Bloodthirsty Yin-Yang Orb? Don't remind me of HRtP, ZUN...