So Spike is sired by Drusilla. But what if something happened to her straight after, leaving Spike alone? She could either get dusted, Angelus and Darla could convince her to ditch Spike, or she gets a vision about him. Either way, Spike - barely even evil with still a lot of William left in him - is left on his own to fend for himself. 

He still tries to sire Anne so he doesn't have to be alone, and killing her still traumatizes him. In response he finds the men who bullied him and kills them all with railroad spikes. He names himself Spike in order to start over, but doesn't really kill anyone or cause much destruction; if he kills and feeds, he only goes after murderers and rapists. 

Over the next century, he begins to fight other demons in order to get his kicks. He hears about Slayers and fights them too, but he never kills them; just fights them because it's fun and challenging. This earns him a reputation of being the only vampire to fight multiple Slayers and survive. 

If he and Angel meet on the submarine during WW2, have it so Spike actively helps Angel and never killed any of the men in the first place. Also have him against the siring of Sam Lawson and is the one to dust him. 

His desire to be loved also leads him to occassionally taking care of anyone who needs it. Fledgling vampires who are like him, harmless demons who need help, and even some humans on occassion. They never stick around for one reason or another. 

He only hears about Buffy after she's taken out the Master. He arrives in Sunnydale around the same time he does in canon. 

Buffy is surprised by this vastly different vampire, and can't help but be drawn to him. Giles is curious, since Spike only fights Slayers and never kills them. Angel is annoyed, since Spike is drawing Buffy's attention away from him; he tries to tell Buffy that Spike doesn't have a soul and is therefore evil, but when Spike continues to prove that view wrong, Buffy begins to question who Angel is. 

Either a new Big Bad shows up or Angelus returns. If it's the latter, have it so Angel loses his soul another way (sleeps with someone else).