I'm sure everything could be modularized out as resource, including levels -- but that would probably end up making the frame editor a lot more generalized as a tile editor if you were gonna do that. As a solution, in MMF my approach was to re-impliment a tile loader from scratch to allow for user-created levels for my game. Many MMF games use that now, but it's not exactly the most easy thing to impliment.
It really depends on the sort of tile-based (or whatever) solution you end up making for your game, because in a lot of these cases I'd wonder why not just release the entire source file to the game if you don't want to impliment your own tile engine.....
Alternatively, we could also have a tile blitter extension on top of the current coordinate-based system which makes tile-based games a lot faster to implement these sort of levels made with dedicated external tile editors, or "in-game editing" of the tiles ala Sonic debug mode, but hey, if we're gonna do that why not also implement a text-blitter for graphical fonts too while we're at it, and then we wouldn't need to include vector font files in the first place XD
I like your guys ideas :V