Theories are fun. I was reading one at Whedonesque the other day where someone saying Amy was maybe The First.
I don't know though, The First Slayer as the First Evil, I think that might be hard to find support for.
The First Slayer does appear in Season 7. In Get it done she's in a Buffy dream where she tumbles down the stairs wrestling with Buffy and tells the Slayer "It's not enough".
Assuming the First Slayer is somehow able to contact this reality from wherever she is, why would she want to? What's in it for her if the watchers, Faith and the potentials die, Spike is corrupted, Buffy is brought over to her side, and the scythe is activated?
What's the disruption in the slayer line Beljoxa's Eye speaks of? Is that how the First Slayer is unleashed? How does an all-knowing entity like Beljoxa's eye get the First Slayer mixed up with the First evil? Is the First Slayer working through the First Evil somehow?
What were calling The First Evil does tell Caleb it wants to be made corporeal. Is that the first slayer's game plan too?
The Shadowmen and the Guardian talk about the last guardian of the hellmouth, and the final battle. What's that all about? Are the Shadowmen in league with the First Slayer?
Oh wait, I just thought of a good one. At the end of Lessons the big bads talk about going back to the beginning.