I've had a normal villager move out four days ago, and no new plot has appeared. Usually when a villager of mine moved out, two days pass and a new plot appears, or a camper the day after they move. I have eight villagers now, and I do have space for a new villager, so what's happening? Is it because of upcoming events that were in the way? (Harvest moon, and now the bug off) or do I have to wait a little bit longer?
When they move in is a bit random, I've usually had someone move in the day after but sometimes it's taken a day or two for villagers to show up. If the camper villager moved out you'll need to watch the camp, they won't auto-move in I don't think, and campers can take a few days to show up (they're very random).
Note that, at least in past Animal Crossing games, the rating of your town affected how many villagers you get. Try talking to Isabelle to see your citizen satisfaction; if it's not high enough it's possible the game is preventing more villagers from moving in. I'm not sure if this is still the case in ACNL (couldn't find conclusive info either way) but older AC games did something like this.
Villagers' "Moving In" Time is basically picked at random. Plots will appear the day before they actually move in. After that, give 'em a day to unpack.