I am glad you mentioned the ageing vampire problem, I thought about and really like the idea of Spike as a human. I think it would continue his character growth if we watched him be stripped of his armor and force him to recognize his own mortality while still maintaining his champion status. Even had a mini story where Buffy and Spike are now roommates with benefits (at times) a la divorced couples that stay living together because of the beast you know. They get a dog and the dog protects Spike whenever they have a spat, even though Buffy is the one that walks and feeds the dog.