I wouldn't need much. I'd like to see him train, on his own and with Giles, and even Buffy (getting less defensive about that). I'd have liked to have seen more exposure to his family where Xander gets ripped into by his parents (as he surely does), and have it dawn on him that his ripping into Buffy and others is imitating that behavior, so strives to overcome it. (Arguably, it happened off screen.)
I won't say the tragic arcs and endings I can think for him...which is not based on hostility toward him. I love Jenny, but I was satisfied with what we got, and it drove the horror and stakes (no pun intended) home. He's self-sacrificing a lot, even risking his life to save people he doesn't like (like when he raced through fire with Cordelia, but then basically told her to get lost when she came to thank him later), so I'd like his tragic end to have an aroma of nobility to it, comparable to Buffy at the end of season 5, at least leaning in that direction (at the very least, Doyle level).
While I don't really want it in the show, if I were inclined to still write fanfics, I think I would do one of him as a powerful warlock (which I can tap canon for), with even more power than Willow. And rather than hubris, his really can follow the addiction story line given that he comes from a family of addicts (*) and drives home that no matter how much power you get (money, physical, magic, whatever), it doesn't miraculously make you happy or drive away the demons (his parents) in his head. (Maybe his overcoming his childhood could even cure him of being a demon magnet...).
In this case, I can't decide whether Willow is still much of a witch, or if she instead focuses on technology. Otherwise, anything more than a short story begs for a conflict between them (much in the style of Buffy and Faith). It keeps them from being redundant if she's tech-gal. (And kinda fits with the Mulder and Scully formula, a nice reverse where the man is the mystic and the woman is the skeptical--but not foolish and dogmatic--scientist.)
(* I was raised for years with alcoholic parents and been around addicts to stronger drugs in my life, but other than some experimentation, I did not follow the path of addiction...at least not chemical anyway. That is, I'm not operating on the assumption that a child of addict will grow to become an addict, just that I think that scenario would've worked better with Xander than with Willow.)