I don't know if they started out with that much of a longterm plan, I always felt like because it was a spinoff & they wanted to make it different from Buffy, they felt there way through season 1 as they figured out what the show was going to be. I think that, if it wasn't for the bts problems, Doyle would have stuck around longterm, definitely through to season 4 like you think, because that's when the show started to get a lot darker. But up until that point, I think he would have been as important as Cordelia had always been, & I didn't really expect them to ever kill her off
