1. A body-switching episode, either just two of the characters, or "musical chairs" like in that great Farscape ep, "Out of Their Minds."
2. Another AU ep like the Wishverse, set in any season--S5, 6, 7. Buffy is "visiting" the other universe like Cordy did, but with more conscious awareness; Spike and Xander are together there; and Buffy takes back info that helps with the big bad arc (e.g., like finding out some useful info about Glory). I like the idea of S/X being done very casually, as a matter-of-fact sidenote that would nonetheless establish the potential for it, if circumstances were different.
It would also have been cool to see an AU set in S6 or S7 where we "reset" the universe and all traces of Dawn are removed; like, what would have happened to the kids if she'd actually never been there.
3. An S7 ep that works in a bonding/buddy plot for Spike and Xander, where Spike helps Xander somehow, either through some act of bravery, or in a more personal way, and Xander actively sheds a lot of his issues with Spike, in the same way that Buffy does--while remaining true to the character's issues and vamp hatred, of course.
4. Any ep after S4 where we see a hint of Giles's personal life, the ramifications of his choices, etc. An ep set in England where we see Giles and maybe Willow working with the watchers--like, it would have been cool if, instead of those few scenes in the country in "Lessons," we'd been shown Giles and Will at the council headquarters, and given signs that they'd established personal relationships with some of the watchers. It would have made the destruction of the council later on more shocking and poignant.
5. An ep or two where the entire gang gets sucked into a portal and has a Pylea-like mission to get home, with swords & sorcery & such. (Have Willow reference Pylea, because she's been corresponding with the A.I. gang. That helps reduce the rip-off factor, gives some great tie-in, etc.) Set it in S6, have Riley come along, and you could rework an entire section of the season, probably between "Gone" and "As You Were."