Were I designing one from the ground up, I'd include a bunch of customizable "archetypes" for the players to choose from- essentially pregens that you can muck about with.
If the idea is to allow for flexible continuity of characters from session to session with minimal effort, then I would drop the scheme of characters getting ever greater pluses to this and that.
Forget about everyone happening to have on hand characters of level N -- what if they aren't hauling around character records at all? If there's no need to remember a lot of game-mechanical stuff, then it could be easy to "reconstruct" a character with what's in the game set. The really important distinctions would be in the character of the character, which should be as memorable as it is distinctive.
Thus, one could have a very episodic game that nonetheless allows for recurring characters. Scenarios could include roles of general types, playable either with the individuals indicated or with some or all key ones replaced with players' continuing characters.