New campaign idea - PCs wake up with no memory...

It's a well-used cliche I know but I'm putting ideas together for a brand new campaign for a new group of players.

I was toying with the idea that the group woke up in some unusual environment with no memory of how they got there or even who they are or what their skills and abilities are. I was thinking of giving the players a bare minimum of information in an extremely cut-down scoresheet and they'd spend the first few sessions trying to come to terms with who they actually are, how they know each other and how they came to end up in the situation they all woke up to find themselves in.

The scenario they wake up to is that they wake up in the center of a magical summoning circle in a mages tower. As they look around, they discover that whoever it was that was responsible for performing the ritual has been massacred by something big and nasty but that none of the characters have been injured in any way (as if the circle protected them).

I am going to treat the characters as if they were first level, and as they try new things (fighting with weapons, armor, attemting spells, etc.) they will define their class and, at some point, I will grant them enough XP to level up to 2nd level, at which point they'll get a proper character sheet. As part of this, I am considering "skinning" the whole levelling up process as them remembering (or relearning) previously known skills and abilities. I forsee the majority of the player characters ending up multi-classed or as hybrid builds.

The whole basis of the campaign will be based around them trying to discover their true identities and what events led up to how they woke up where they did.

What do you guys think? Can characters evolve during the course of the first few sessions or do they need to be pre-defined? My concern is that players are used to coming up with a complete character concept including background themselves. Could it work?