Spike goes through the broken back/Drusilla leaving him bit in a different way - any way other than Buffy causing it. Maybe the mob in Prague got him injured? OR Dru abandons him because he got chipped early? Or both!
Anyway, he comes in around the time of School Hard to get revenge on Drusilla, who he heard was coming to the Hellmouth to get back with Angel. Angel turns into Angelus (could be by Dru or the canon way with Buffy.) Either way, Spike & Buffy team up to take them down and Spuffy goodness happens.
I want Spike to already be close to the side of good, having decided that the way he was "raised" by Angelus/Darla/Dru was horrible and they manipulated him into becoming the monster he was. Maybe Dru used her thrall on him in addition to Angelus torturing him into being a "better demon." Since his break with Dru he is now free to live life how he wants. If he is NOT chipped, maybe he feeds on rapists/murderers/etc. The way he acts without a soul makes Buffy & Co. question the value of the soul and what it really means. Have the Scoobies explore and question how Spike is sort of amoral but chooses to try to do good because it feels good. Let's take a look at the nature/nurture thing and also if how one acts is a better indicator of their "goodness" than having a soul/being a demon.
My one absolute rule is that Buffy cannot be cruel to Spike. Snarkiness is fine, though. It is Spuffy, after all. :)