Nice idea for a fan fic epic by the sound of it. S1 has got plenty of scope for lots of great character development. There's actually a thread about what people would include if they'd had the chance to put in some extra episodes. You could check it out for some ideas.
I'll comment on each element you out-lined individually:
- The Master would be more involved in the series, as he would often taunt Buffy in her dreams before a new 'Monster of the Week' arrives.
It's good that you'll have more Master/Buffy interaction. The villain and the hero should always have a few back and forths before their final battle.
I'm not sure about the dream-taunting though, especially if its over-used it could get tired. Maybe limit this to once or twice and have an explanation as to why he can do it at all. Star alignment, magical artefact, blood ritual etc.
Maybe have something a bit like 'Nightmares', with Buffy is getting stalked by The Master in her dreams- all a big scheme to kill her and/or break free of his prison.
Or maybe have an episode where The Master has a scheme to escape his prison (like he tried with the Harvest) and have Buffy go down to the church to strengthen the barrier somehow. Have them faceoff but not fight just yet (Buffy fights some of him minions or something).
- Jesse would be alive for the first half of the season, making more of an impact when he is killed.
Good. Jesse getting killed (and turned) later on would up the stakes. Also make more of his friendship with Xander perhaps, explain why Xander really hates all vampires, even Angel.
- Spike would be hired by The Master to hunt, and possibly kill Buffy.
Eh. This one I'm less sure on.
Soulless Spike is a lot like Angelus this way, I just can't see him being 'ordered around' by a bunch of religious types (we saw how Spike viewed 'the Anointed One'), he tends to be (or want to be) his own boss.
Plus isn't the Master enough of a badass without calling in some 100+ wet-behind the ears vamp?
Why not stick with Darla? Make her a bit more formidable (like she was on Angel) and play around with the Buffy/Angel/Darla dynamic. Or have a few more of the Order fleshed out- like Luke was.
- Giles used to train the Slayer who was before Buffy, which is the reason he is so ''protective'' of her.
I like this to. Gives Giles a bit more depth right from the off. Buffy doesn't know about the slayer before her, she finds out somehow and this leads to angst/questioning her role as the slayer before Prophecy Girl (or whatever climax you're using).
Good luck!
🙂