Considering that you're completely coding the application from scratch, wouldn't it be better to make it your own IP to avoid legal issues?
PS: if you actually make this a thing to the same extent of Graal with playerworlds, please actually reward developers that put time into making great worlds. The biggest problem with Graal is that developers couldn't be damned to get anything done due to lack of compensation, and as a result there's a lack of content.
Having a form of system like Sony's player workshop that gives players a portion of sales of their content can help encourage talent to dedicate their time to improving the platform, while you focus on improving the client itself.