Here are my rather long thoughts.
Buffy.
With Willow gone and Cordelia staying, Joss wanted this show to have a Gay Character. In all likelihood his attention would have turned to Xander.
With Xander finding the 'Inner Homo', Larry would not have died in the season 3 Finale, infact Larry would have become a friend, even maybe a Part time Scoobie as we saw his potential in The Wish.
Xander would NOT embark on a relationship with Larry, though he'd find someone who he would fall for amongst the sudents of Sunnydale U.
Larry meanwhile, would begin to fall for Xander but his unrequited love would have him fall foul of the scoobies by season 6 having him become a member of a slightly different Trio...sans Warren.
Cordelia would stick around for part of season 4, ala OZ, but realise that she wants more from Life.
She would leave the show, where I believe she would still end up in LA.
Buffy meanwhile would now be left without her best friend, so in would come another female character. Possibly even Anya still, written slightly differently, Buffy now has the responsibility of helping Anya adjust to the world and they become firm friends.
Dawn is still brought to the group as Buffy is the only one that could protect her, but Buffy doesn't die at the end of season 5, but a major Character does, possibly even Anya.
During the next season, Buffy is distraught that in the last year she lost Riley to the Initiative, her Mom to Cancer and Anya to Glory that she begins an illicit affair with Spike.
This progresses pretty much the same way as the real world, with Buffy closing herself off from the world because she is tired of losing the people she loves.
Dawn, in this senario, is the main focus of power this season.
Risidual energy from the key gives her power of some kind, that finally erupts in some major way when Larry accidently kills the love of Xanders life, who is also the good friend and parental figure of Dawn.
She doesn't go on a rampage to destroy the world, but she does try to kill thye trio.
Xander is still the one who saves the day this time, as he, still mourning, convinces Dawn that although he now hates Larry for what he has done, what Dawn is doing is not the answer either.
Buffy sees that Dawn is now a powerful woman and starts to treat her as such, along with accepting her place in the world again, as she realises that Dawn is there and needs her.
Spike still goes off to find his soul.
Giles meanwhile, left at the end of season 5 because he felt that Buffy needed to do things on her own, which unintentionally added to Buffy feeling lost, but ultimately led her to find her strength.
He comes back when he realisesthat Dawn is this seasons 'Big Bad'.
Season 7 focuses on the Origins of the Slayer.
The First is back only now its not because of the Slayer line being changed at the beginning of season 6, its part of a predestined plan to take over the world, thanks to Buffy dying then being ressurected in PG.
This season plays out similar, only Giles stayed in Sunnydale to tutor Dawn to deal with her powers.
Angel still returns with the Amulet that saves the Day, though the potentials that are alive are still without their powers, they do learn to fight pretty well as the rest of the scoobies and become formidible in their own right...
Angel coming soon.
The Watcher agrees: Hmmm. that actually sounds like a pretty good alternative outcome, although you didn't really mention how you though Cordy would fit into the mix.
Not bad for an off the cuff comment eh?
OK, so Cordelia.
Her family still lose their wealth, but Cordelia has a certain level of intelligence to fall back on, so wins a scholarship to UCSunnydale.
However, with her family poor and some of her friends dead from the Mayors attack, Cordy is feeling more alone than ever before.
Xander begins to look appealing again.
But the guys burgeoning homosexuality puts a halt to her chances, so with nothing left to tie her to anything in Sunnydale, Cordelia decides to start a fresh, going to LA in search of fame, fortune and hope.
Her last act however, is one of kindness, as she helps Xander come to the realisation that he is gay.
With that, the two now have a deeper friendship that may come in handy later on
🙂
Over in Angel.
After the big fight with the Mayor, Willow realised that she had more reason to get the best education she could, so with a heavy heart the woman left for LA to study Law.
While at the school, she is approached by a Lawyer for the firm Wolfram and Hart, as a potential for their Scholarship program.
However, local business man, Russell Winters has taken a liking to this woman too. Russell wants her to be on his pay roll, but thanks to some information from from Angel, she realises what he is and that her place is in the fight against the darkness.
While staying at school studying Law, and also taking on a position in the mail room of W&H, Willow also spends her evenings with Angel and his new companion Doyle.
Doyle and Willow become pretty close friends, however in a really sad episode Doyle sacrifices himself to save the lives of a Demon family.
Willow is also having a troubling time, as it has become more evident that W&H are in fact an evil Law Firm.
wanna know something - this one is far harder than the Buffy one.
I'm gonna need a bit to think about this
🙂