That has been tried before with XT and it failed.

Bitcoin Classic is the attempt to change the reference implementation from the Core code to their code, by only changing as little as possible (that is, raising the 1 MB blocksize to 2 MB).

The current problem is not being more open to other developers but to get a different reference implementation installed on full nodes and miners.

Yeah, but that's a chicken and egg problem because unless/until BitcoinClassic outcompetes the other one, none of your code will ever get executed, so any developer wanting to scratch an itch must contribute to the existing bitcoin rather than BitcoinClassic.

The dynamics don't work the same way as a traditonal FOSS fork. Perhaps getting mindshare first is the way.