TwoSix
Everyone's literal second-favorite poster
It's tough to say. For one of my recent characters, I homebrewed a custom subclass, made it look pretty in GMBinder, and the whole thing took about an hour? Not a big struggle.Creating a whole new game is a lot of work, as you suggest.
However, creating new house rules or small subsystems for an existing system is less work, because you can leverage the design that's already been done. You aren't designing the whole thing from scratch, you are expanding in minor ways upon the base system.
But realistically, out of the 20 people I play with regularly, maybe 5 of them would have the combination of game knowledge and tech savvy to be able to do that? The rest are dependent on the material in the game books or have to talk to myself (or another DM) and hope they're willing to work with them.