Dynamic Content: what are my options?

I realize this may be a better topic for a forum, but hey, it's a question: I need to make an app that loads content dynamically. Ideally, it would allow folks with Unity Basic (free), or perhaps some other modeller/IDE, to produce content which would be loaded/displayed by my app.

Here are some options I've come up with, are there others?

1. Make a scene loader, like X3D/VRML. This is very hard (I've already started it), because it would be difficult or impossible to implement scripting and protos (smart objects basically)

2. Use asset bundles. This would be perfect, if it weren't for needing content developers to shell out the $1500 to make a bundle. We really really want this open to as many people as possible to provide content. Did I mention, we pay for content?

3. Some kinda hack at the Resources folder? You can load resources on-the-fly, but I assume they have to be 'compiled' into the game. Would it be possible to replace one compiled resource with another, if the contents adhere to a naming convention or something?

4. DLL's : We could publish an API and let folks compile (using free Visual Studio) DLL's containing their content. But this again would require the Pro version no? Or would a .NET assembly work?

Any thoughts or tips greatly appreciated.