I wish we had access to the Sandbox source code. There's like 12309019231209 features I want to add via blueprint.
He should consider doing a partial open source to github, share non-critical code so the community can help speed things along. He can make a special license where the only thing people are allowed to do with the code is just modify it to make the game better, that's it, no commercial use nothing.
He could allocate all his coders for single player story, but then let the community work on the sandbox. All the core sandbox elements are pretty much done, just let the hobbyists do the rest of the work. The devs already did all the heavy lifting for sandbox, and they did it very well!