This may be insane of me, but I'm so happy with the response, that I don't want to turn anyone away. I've already done much of the prep-work, so I don't think that it would be too much work if I simply....
RUN TWO GROUPS (of five).
This will be two different realities (as opposed to, say, running groups that hit the place from the front and back, as I did years ago in Vault of the Dracolich). So we'll have to form two fully-functioning parties. We'll make characters, and then see who fits best in what group.
I don't have much more notes for you on PC-building rules, other than to say, "Please don't make characters that have a lot of fiddly tracking bits, complicated spells, OP combos, or other things that will grind the game down." (As to what those might be, I'll leave it up to your judgement. Just remember that PBP can be slow, and a lot of work for the DM, so try not to make it slower/more work. THANKS!)
Oh, and an in-story note: I imagine the kingdom to be mostly human. While I'm technically allowing you to play ANY race-class combo you'd like, I'd probably rather that the parties don't wind up too much a bunch of weirdos or mutant animals. (A few are fine! In fact, I want a few! It would just be weird if it was the whole party!)
Also, feel free to go hogwild with re-skinning. Mechanically, keep it simple (no houserules or weird corner-cases), but if you want to play a Minotaur Barbarian with the Charger feat, and make up a Rhinoceros-headed person to explain it's story, you do that.
The setting will evolve (though not revolve) around your characters.
Oh! One more thing: Ninth level is pretty high, and I imagine this to be a relatively low-success-rate setting (life is hard, no one is getting ahead). So, you're pretty special people. Like, probably pretty well known.
Also: You don't have to come up with long back-stories (unless you love to do so, then you do you). You'll probably die anway. Just make a fun character with some broad-stroke ideas as to who you are and why you're helping out, and you can figure out the rest as we go along. I'm pretty happy with making some stuff up as we go. (Also, I'm fine with leaving stuff that doesn't matter... not mattering.)
Thanks for the great response! Let's get some character ideas flowing!