I always kind of wished at the end of the series he would have been the primary vessel for the First... just to see some semblance of him being the Big Bad again.
God, I would've hated that... One of the best things about Spike is that he made his own decisions rather than being slave to the fate and whatever, which is why Joss calls him a more evolved character than Angel who was cursed with a soul, and says he always liked him better... The last thing I'd want to see is some stupid possession story. Yeah, he was controlled by the First for a while, but ultimately he was able to fight back, which is more like Spike.
One of the things that people tend to forget is that Spike was the Big Bad only for half a season... He was meant to be the decoy Big Bad and die in "What's My Line pt 2" - Angelus was always meant to be the real Big Bad of season 2. But since Spike became so popular, they decided to keep him alive and just crippled him temporarily... And then of course there was the great turnaround and Spike as the unexpected unlikely ally. Of course he was just the 'lesser evil' at that point. In S3 they brought him back as a comic relief more than anything, in S4 he was a real villain for 1 episode when he was trying to kill Buffy and get the Gem of Amarra (and 1 other on AtS), before he got a chip. Which they had to do in order to make him a regular, so he could interact with Buffy and the Scoobies without trying to kill them. And then he was just comic relief 'neutered vampire', until he fell in love with Buffy in S5, at which point he started being treated more seriously again and got more complexity and backstory. He was never going to stay on the show for long as a villain, since his main goal was to kill the Slayer, and you can't kill Buffy since it's her show, so if he had kept trying to kill her, it would've made him look incredibly ineffectual, he'd be like Wile E. Coyote.
So, all in all, Spike spent just 7 episodes of
Buffy as a real Big Bad (6 in season 2, if we count "Surprise" where he was in wheelchair already while Dru was strong; and 1 in season 4) and 1 more as a proper villain on
Angel (season 1) - all in all, 8 episodes, and in two of those (BtVS season 4 and AtS season 1) he was a fuck-up who managed to screw his own plan on the verge of success. Plus he didn't really do anything special in the villainous sense in early season 2 - although he did beat Buffy in their first fight and would've killed her if it wasn't for Joyce and her axe ("Get the hell away from my daughter!") and he and Dru did bring back the Judge. But the biggest thing he did in season 2 was actually go behind Dru and Angelus' back and ally with Buffy.
It's interesting to note that Spike, whether soulless or souled, has so far (counting season 8) saved the world or contributed to saving the world 4 times, while Angel, whether soulless or souled, has managed to bring about at least 2 apocalypses.
