The title says it all.
Currently, we have two pools - pool #9901, Reisen's ascension, and pool #6960, Sakuya's comeback - that are about the same topic - a character becoming playable for a specific game.
However, there are images of this sort for just about every game in the series, especially relatively uncommon characters, highly requested characters, and so on. For example, post #6588103 of the most recent game would certainly qualify if Orin had a similar pool, as she's been a long-requested playable chararacter, and posts like post #874708 would go for Ten Desires and Youmu.And that's only counting ones where characters are actually confirmed to be playable - there's plenty of posts about characters trying to become playable, or being unhappy that they're not playable, that don't tie into a single game like post #2133010.
Not to mention that the two pools we have are small and basically static at this point, and it's pretty reasonable to approximate them with "character name + game name" as two tags.
What I propose is one pool that covers all playable characters, both their attempts to become playable and celebrations about them being playable in all games.
The reason I think that is the best is that we can use game tags or characters tags to refine the search to a specific instance. For example, if such a pool existed, you could get all of the sakuya's comeback posts by searching for the pool number and Double Dealing Character; if you wanted to see all of Shou's attempts to become playable, you could look up the pool name and shou. you can even extend this to long-desired PC98 vets like Mima. so on and so forth, you could get a reasonable approximation for everyone from one pool.