There's two existing pools about specific characters becoming playable - Reisen's Ascension and Sakuya's comeback. I'm thinking it might be worth making one unified pool that covers the quest to be playable across all characters - then you can just filter to a specific game or specific character to get their appearances.
Don't worry Shou. The more Touhou games come out, the more stage 5s will come to wait in line to be playable. Now besides Shou there are still Clownpiece, Mai and Satono, Mayumi and Megumu that are yet to be playable. Shou is the only one from older era to be unplayable yet. How sad.....