On a side note: We're already working on a similar project but focussed on Apollo. If you're interested in getting involved, we're still looking for collaborators: https://github.com/learnapollo
Have you also seen the integrated GraphiQL and data browser?
When thinking about building a GraphQL entry point there are 3 main areas you will have to consider: Performance, Authorization and Complexity.
GraphQL is designed to be backed by arbitrary storage, so it would actually be feasible to have a bunch of GraphQL resolve functions that just call out to existing REST endpoints. To make this performant you need to implement a proper batching mechanism like DataLoader in node or Sangrias Deferred Resolver. If your existing api has automatically generated docs in a format like swagger you might even be able to generate the entire GraphQL schema from this :-)