"When do rare mobs in vanilla WoW respawn? Do they respawn at the same time every day? Do they respawn after a server restart? Are there any exceptionally oddball rare mobs (say, once every 4 days)?"
As far as I know, there's no rule to how rare mobs as a whole work. Each one has its own little respawn rules, though I somewhat have my doubts that any of the rare mobs in Vanilla WoW are on a 4-day timer. Though I suppose there's a chance of that, I don't know. If you're hunting down a specific rare mob, I suggest just going to Wowhead and reading the comments for that particular mob. That will serve you much better than trying to hunt down one solid rule for them all. There simply isn't one.
"While we're on the subject, do you think the other faction leaders will ever get their own unique models, like Sylvanas and Velen?"
This is something they talked about during the Wrath of the Lich King beta, when they pushed through the changes for Varian and Sylvanas. It was on the beta forums though, so I'm not sure I would be able to find it again. Essentially, what was said was they plan to update all of the racial leaders, but it will happen gradually and probably when those leaders are filling a big role in the story.
If Thrall is ever actually leading the Horde again and not letting Garrosh pee in his Cheerios all day, Thrall will probably get his upgrade. He really needs it, too. Both the upgrade and the stop-peeing-in-my-Cheerios attitude, really. It would be nice to have Thrall in charge again.
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