I know what you mean and I have thought about that kind of thing. However, I for example would like to do something completely different. I have originally studied maths, so I would like to do stuff that requires a lot of maths.
My current jobs involves things like increasing code coverage of unit tests or fixing CSS stylesheets. Sure, I could come up with a new CSS framework or something, but there is not actually that much time allocated for each task.
Most brain-rot inducing jobs are characterized by repetition. Depending upon where your job is mired, try doing the following:
* Making libraries / frameworks to speed up tasks * Making scripts to speed up tasks * Hooking scripts to forms * Making forms user-available ("self serve")
At minimum, building these efficiencies while still doing the boring stuff will be a _slight_ challenge. And when you are done you may actually have tools that you can productitize.
At minimum you will have the experience.