3) On new character(s): ZUN said you can't just add random characters to the game. He might add someone who makes people suspect "something has begun."
Based on this, i'll do the process of elimination. I'm almost sure that ZUN will do something new (story-wise), so he won't recycle old plots.
Ignoring the already discussed Yukari and Nue, i don't know if the PS4 version will be simultaneous with the PC version (so no LoLK characters because they weren't introduced yet) or the new material happens later, Hisoutensoku-style (a chance for LoLK characters)
*I consider that the residents of the SDM and Hakugyokurou are too well-stablished
*Team ⑨ is full of bakas.
*Letty couldn't work in a long plot because is only active on the winter.
*From Eientei: Eirin is a mastermind, someone who would send others to fight, Kaguya is a
NEET refined princess that wouldn't fight by herself (unless we are talking about Mokou), Tewi is too minor.
*Suika is just a drunkard.
*Yuuka and Medicine are too absorbed in their hobbies.
*The innumerable gods of Youkai Mountain wouldn't influence in a major plot
*Aya and Hatate would work better doing the generic work of
paparazzi journalism, but not being involved in a notable incident. I don't see Momiji as someone who would go out to beat people in a incident.
*The Moriya shrine could plot a new conspiracy, but it would almost certainly be on the excuse of gathering faith, too predictable.
*Tenshi is an spoiled girl and Iku her nanny. They wouldn't attempt something new.
*The underground residents are in their own secluded world, not much of a reason to go out in Gensokyo. (except for Koishi because she's Koishi)
*If they need output from the Buddhist and Taoist factions, they have the already-present playable characters.
*Kogasa's modus vivendy is (attempting) to scare people. A simple girl with simple motivations, but too predictable.
*Waggysaggy couldn't work properly out of the water
*Nobody cares for the Tsukumos.
*Seija already has her own game, and simple motivations (too bad, her contrariness could be comedy gold).
*Aside of Kasen, the manga-intruduced characters don't have the physical or magical prowess to fight, nor too much motivation to pick up fights on people on the level of the already-seen characters (eg. Byakuren, Miko, Reimu, Marisa) as if it was nothing. Except for the moonbitches, they are too OP.
My predictions:
Keine (low probability, could do something to protect humans but not much), Reisen (the one that does all the heavy work for Eientei), Komachi/Eiki (low for Komachi as she was already playable in a fighter game, better probability for Eiki, who could do something in reference to the newly-introduced Hecatia), Seiga (low probability, seen by others as 2evil4u, could have some fun for herself), Sekibanki/Kagerou (Moderate probability, they are relatively new and fresh, they could do something to scare off the human population).