Yeah, I think the idea of a free and a licensed version is good enough. But for me, the problem there is the price for the licensed version <img src="smileys/smiley12.gif" border="0" align="middle" /> . I also like the idea of selling individual plugins or something for you to add to your free version just in case you don't want to have all the functions in the licensed version (depending if that system is going to work anyway).
For example, you have a free version and it has all the functions you need for your game except for one plugin which we'll call PluginA. Then you know that PluginA is included in the licensed version. So you can like buy only the PluginA and import it on your free version and you're all set to go. Something like that I guess.
An a la carte system sounds interesting, but I wonder how that would fit into the indie, and pro licensing.
Plus there's the thing about a dev kit for making plugs.
I guess they could disable importing of plugs for the free version. But that opens up a whole other can of worms.