Chet Faliszek and Eric Wolpaw (Both story authors at Valve) have made a presentation about the original plans and early concepts of Portal 2.
They talk about the different, original beginning, original endings, cut competetive multiplayer, the several other characters that should’ve appeared in the early Portal 2 (Like the Morgan Freeman sphere), changes about the game overall, why they cut Chell, GlaDOS and portals from the original plans for the sequel and much more.
https://www.youtube.com/watch?v=OLqk4aqpXlQ&feature=player_embedded
It’s one hour long, but really interesting (and funny too), as it also shows, how Valve actually works. (With their playtesting and how they work on their projects)
And there is an Q&A at the end of the presentation.
I hope you enjoy it.