The Part 15 FM transmitters pose a challenge for the ambitious engineer hoping to improve them with modifications.
A modification might be, for example, adding a switch to turn off the stereo generator.
Looking at five transmitters on hand, it's mostly the IC chips, the integrated circuits, which raise the greatest obstacle for the most part because some of them are unmarked and can't be traced to a data sheet which would aid in their reverse engineering.
It would also be nice to add a control to calibrate the stereo injection level, many transmitters are found to have stereo pilots or subcarriers set above professional levels.
I'll sit back while someone makes the meaningless claim that modifying some of these devices negates their certification. Then I'll come back and explain why that's a non-issue.