Alright, two more ideas:
A goldfish youkai. The joke around her is the fact that goldfishes have memory spans of only a few seconds. She'd be like Dory from Finding Nemo: forgetting everything after only a few seconds. She would even forget that you just kicked her ass a few moments ago. This could provide quite some jokes. If you want to get extreme she'll even repeat some of the lines she said, forgetting that she said them at all. So then you get: "Oh what a cute Shrine Maiden! .... Oh, what a cute Shrine Maiden! .... Oh what a cute Shrine Maiden! .... Oh, what a cute Shrine Maiden!" etc. Reimu would probably attack her for annoyingly repeating everything she said a few seconds ago.
Now the other idea, and I need to explain a few things before I can tell the actual idea:
Have you even wondered what happened to the previous Shrine Maiden? Reimu's mother? Why isn't she around the shrine to help her child? Why did she pass over her task to Reimu so early? Shrines usually have multiple mikos, and the lead miko tends to stay there for a loooong time. Why would Reimu's mother spontaneously abandon her job and leave it to her (at-the-time) ten year old daughter, knowing what kind of dangers a Shrine Maiden needs to face? Isn't that irresponsible and uncaring?
I think there is a good reason why Reimu lives alone at the Shrine from such a young age.
Her mother was killed. By a youkai.
Because of that Reimu had to take over the Shrine Maidens job from an early age, living by herself for all those years. There had to be a new Shrine Maiden, or else the Border would collapse. This might explain why classic Reimu was so incredibly mean spirited in the PC-98 games, sealing youkai for hardly any reason: she held a grudge against them. They took away her mother, and now she grinds them to dust at the smallest sign of provocation. In the windows series she seems to have slightly grown over it, but still lets herself be consumed by these urges from time to time (attacking Tokiko for example). This might just explain all those 'what the hell, Reimu' moments. And now here comes the character concept:
The youkai who killed Reimu's mother. The first, truly evil Touhou.
This youkai might have hidden itself for all these years and only just now reemerged, or Reimu knew where he/she was all along, but just wasn't strong enough to face him/her yet. Alternatively: Reimu's mother sacrificed herself to seal this youkai up, or even destroy it. But now its broken free/reincarnated, and it's out for revenge, hell bent to destroy the Hakurei bloodline. This dude is going to be bad. Very bad. Bordering on complete monster. And if it managed to finish off a mature Hakurei Maiden it's going to be strong and legendary. I think the being most fitting for this role is none other than Yamato-no-Orochi. It might be the very final villain, or it might not (I think Amatsu-Mikaboshi will be the villain of the real final game), but its going to be a historic battle for Touhou Project.