Instead of getting infected by a psychic demon in season 3, Buffy gets infected in season 5 or 6 (possibly 4, but I think that it would work best in 5 or 6).  She has a rude awakening when her friends have some disturbing thoughts (how disturbing is up to you) and she finds out that only one person truly has her back.

Must:

-She can hear Spike's thoughts, none of this "mirror, so no thoughts" BS.

-Willow is starting to get deep into the magics and has a bunch of thoughts on how to "fix" Buffy.

-Xander can either be still obsessed with Buffy or just be very anti-non-human, but he still has to be within the cannon, not too ooc.  He loves Anya but wants to deny that she was anything else before.

-Giles loves Buffy but still has reluctance to treat her like an adult.  If in season 6, he thinks that it was his failure as a strict Watcher that caused her death, and if he's too lenient, she'll die again.

-Tara and Anya want Buffy to be herself, but they have loyalty to their significant others, so rarely speak up for her.

-Spike, of course, does everything he can to help and heal Buffy.

-They (the Scoobies) can either make up at the end, or Buffy does her own thing, but HEA, please.

Possible:

-If a longer story, have an overarching plot like the lunch lady, a person who needs help that they're trying to find while helping Buffy.

-Riley can exist, but get rid of him quickly, please.

-If Angel shows up, he doesn't know about her ability and can't counter it, so he also has some disturbing thoughts that Buffy needs to come to terms with.

-Buffy staying in Spike's crypt to get away from people.

No:

-Cheating.  If Riley exists, he has to be gone before Buffy and Spike get together.