Jenny for Joyce - it would have made for a very compelling storyline and kept that horrid Bangel from degrading in S3 and souring me forever to the pairing, lol. But, really, taking away Buffy's mother, very interesting concept. I chose it for that, not cause I love Jenny more than Joyce, promise.
Buffy for Dawn - like a lot of others here, I would have love to have seen a S6 where Dawn made the sacrifice in The Gift, how that altered everything. Did they keep their memories or not? Did they only remember Dawn as she had been, from her arrival in Buffy vs Dracula? So many opportunities for interesting story telling.
Tara for Willow - ouch, so sorry (specially for the ass kicking WFB will give me) but again, the story telling aspect; Willow dies before she can fully redeem herself for her magical actions (but also without going fully dark, yay?) and now Tara, a much different person from her girlfriend, has to deal with the grief, watch the others grieve, try to take on the mantle of head witch for the gang, so much beauty to be told with this story.
(and a fun one for my silly heart) Spike & Anya for Angel & Wood - Buffy would give the amulet to Angel, knowing it would probably kill him, because she didn't want Spike to die (like I said, silly) and Wood would be paired with Andrew and slashed by that Bringer. (in this version, Amanda also survives and Kennedy dies 😀)