I’m not sure you even need to involve money for this, it could just be a platform of joint feature requests and development, which results in PRs and forked branches, and everyone can just merge these into their own engine build until they are accepted by Epic.
There are many of us already committing a lot of improvements to the engine without expecting anything in return, and I don’t see a reason for it to change. Having a board of doable feature requests, organized by vote count, extended with a discussion board for developers to discuss the best way to achieve it, etc, sounds good enough to me.
As soon as you involve money, it changes the community-driven idea into a business to sustain, with money handling, legals, privacy, taxes, and a bunch of other issues.
Anyways I also believe Answer Hub is not the forum to discuss this topic.