Hmm, I propose the following:
Marisa: Patchouli, Alice, Nitori(Something like a secret character that is hard to approach), Reimu
Sanae: Kanako, Suwako, Reimu
Reimu: Sanae, Marisa, Remilia
Sakuya: Remilia, China
Reisen: Kaguya, Eirin, Tewi, Mokou(extra-hard to get character)
Could be interesting if Rinnosuke was a character all girls could get, but was extra hard to romance, and required much effort.
These are what I can think of right now. I'm up for writing scenarios if needed.
Not much of this is needed to be finished in the next update, since it's a demo and all.
It would be nice if characters like Flandre and Cirno were eligable for "little sister"-like relationships, with them handing you things that are usually useless junk, but from time to time will be something rare.
Also, for the next planned playable character, it would be interesting with Cirno, if you're planning to make another character.