Unity free for creating user levels

Hey,
I would like to know if its possible to load a entire level made by a user in unity and what would it involve.
I've made some code that serializes some of the objects in the level, like terrain, with textures, normal maps,
and trees (predefined prefabs). But everything is already offered by the game or unity (and is referenced usually by name).

What if the user wanted to use scripts? Like can he add a "time of day" dome to his level, use RTP terrain shaders? which isn't offered by the game.
Would this even be possible in pro version asset bundles? From a older thread, shaders and code must
be offered by the game.

Basically I'm trying to make it like UDK, I provide some actors, the user places player spawn actors, and other game actors where he likes and from there he's free to do what he likes and then he exports it to a package/file for the game to load.

Lets say I'll just let them use only assets that are offered by the game and some mesh loading, and I bought and use
Time of day and RTP that I mention above in my game so they could use it to make the levels, and other stuff that really change the look of the map.
Some of those are commercial and aren't free so I can't even redistribute them for use in unity free.
So my guess that I'll have to choose:
1) Just let them change variables for those and they won't see its affect till they really load the map in the game
(A pain for the map maker)
2) Make the game editor inside the game, not a small feat (A pain for the me).




How do I go about this?



Thanks ahead.

 

(UP)