This is something that I observed from the millions of resets I did with the new character trick. I’m pretty certain I’m not the first to write about it though!
I’m not entirely sure how it works with 10 townspeople but with 9, there is definitely something going on in how the game moves in animals.
Here’s my original list of residents in my second town that I use primarily as a base for my wish list characters: Dora, Robin, Nate, Walt, and Agent S. Hippeux, Pierce, Hazel, and Blaire moved in to complete the last 4 spots. Blaire was actually a camper that I convinced to move in. Sadly, aside from Agent S, Hazel, and Blaire, I’ve yet to get the other ones I want. 😦
Anyway, Hazel (an uchi type) was the first to move out. I make note of her personality since it seems to be the determinant for “natural” move-ins (as opposed to ones that you invite from other towns yourself). Edit on September 30th: It seems that residents gained from StreetPass bypass this trend too. 
I didn’t really start noticing a pattern until after Hippeux (a smug type) moved out. Basically, the game absolutely REFUSED to give me any other personality type OTHER THAN uchi. It seriously cycled through almost all the uchi characters. I eventually settled on Mira.
Not long after, Pierce (a jock type) moved out and when I began resetting to find a character I wanted, the game continuously threw smug animals at me. I was hoping to get Graham this way but I accidentally loaded up the game with my mayor and ended up with Ken. 9__9 He seems cool though so I don’t mind.
Dora is the most recent resident to leave. Since a jock left before her, I expect that the next batch the game will be cycling in are of that type.
And it is.
So far, it’s given me Rory and Stinky.
To sum it up — the trend for moving in is based on the personality type of the animal that left previously. Or “second last”. Here’s my experience based on the above:
- Uchi character left
- … can’t remember if it was giving me a certain type but I ended up with ugly Chrissy who’s a peppy type.
- Smug character left
- Uchi character is moved in by the game.
- Jock character left
- Smug character is moved in by the game.
- Normal character left
- Jock character will be moved in by game.
Going by this, after a jock settles in, a normal character will be the next immigrant to my town. :3 As for campers, they don’t seem to follow a trend as I’ve gotten 2 peppy, 1 smug, and 1 lazy so far.
Being able to predict the personality of the next animal to move in is kind of fun because I’ve actually started making a list of the characters I wouldn’t mind from each personality group. XD
For jocks, I’ve selected Hamlet, Poncho, Scoot, Cousteau, and Buck. ^.^ And for the impending normal one moving in, I got my eyes out for Flurry, Kitt, Marina, Merengue, Poppy, and Maple.
Sifting for a character you want is an insanely time-consuming process. I should be happy with whoever moves in — and considering the purpose of this particular town, I shouldn’t even care so much — but there are some seriously hideous designs … like Chrissy. >___< She ended up as a resident because I got lazy and didn’t do another run around the town with a new character before starting the save file with the mayor. I do not like her design since it looks like it was done by someone that failed miserably at drawing an anime-style face. >_>
Woo! Just spotted a plot of land for Poncho! I’m calling it for the night. ^^ His location is quite a ways from the other residents though. I’ve never seen the game put someone so high up on the map for this town! Everyone is either in the south or the west. :0 So strange~!
koukoupuffs 