This comment is exactly what had me worried from the start. The largest challenge is the notion of having different players at the same table playing different forms of the game (basic, standard, specific modules, etc.). I think this is still possible but balance will be a major obstacle. From the playtest, one could have basic players relying largely on attribute checks while standard players use a skill system. The trick is designing the system so that the standard players neither trail behind the basic players, or vice versa. There can be some differences (one would expect skill-based characters to be more focused -- with more power in their foci but at the cost of flexibility) but any dramatic differences in power will lead to problems with mixing models at the same table.
With adventures, one could create single documents based on each model for skills. Descriptions of challenges could include basic(attribute-based) difficulties and standard (skill-based) difficulties. This does not seem too hard to me.
However, I see a HUGE difference between 4e style combat and 1E style combat. Not only is there a difference in the skill set and the AEDU sytem + grids. The scale of the encounters is entirely different. This is where the initial rhetoric of a modular rule set that would let you grab U1 or a 4E module and run it breaks down.
I am intrigued by the possibility of representing characters with different systems. I would be interested in running a system where I used the basic characters for some encounters (intended to play quickly), standard system for some encounters (notably skill based encounters), and advanced system for "boss" fights. If each player has three sheets that are consistent, this could be a fun flexible system.