Just some ideas, some of them already being used though...
- Mokou vs Kaguya (already exist and pretty mainstream)
- Flandre vs Remi (already exist)
- Kanako vs Suwako (already exist)
- Yukari vs Tenshi (already exist)
- Kyouko + Mystia (already exist)
- Suika vs Yuugi, fighting over a bottle of sake maybe
? (like in your 500 subscriber danmakufu commissions 1st place)
- Reimu vs Marisa, RE:imperishable night, maybe?
- Byakuren vs Miko (add Reimu and become a religious war aka. Menreiki Team)
- Nue + Kogasa, prismriver fight
- Koishi vs Kokoro, rivalry stuff maybe?
- Kagerou + Wakasagihime, prismriver fight
- Orin + Chen vs Nazrin, catfight \ :V / !!