You dont even have to push anything forward. Example Redhat's compilation of code or "forked" by CentOS, who were providing just a little more freedom. We all know how that ended.
Just keep in sync with the main project, and keep the annoying/proprietary stuff out.
> You don't get to stay in the club.
Or you become the club. I think LibreOffice has more club going than Oracle's OOo.
I’ve never used Elm, but that’s not how reasonable open source projects work. Red Hat maintains several rather divergent forks of Linux and they’re still in the club. I personally run a fork of Linux that I maintain, and I’m in the club. The only people who attract serious ire from the club are people who distribute out-of-tree modules that play poorly with the rest of the system. Even in that case, no one gets excommunicated, but the upstream kernel makes no particular effort to keep problematic modules working.
The open-source ethos means welcoming the friendly competition that comes from a fork. Look at the grandparent's link for the attitude taken by awk and bash.
It's a bit like Brexit. You don't get to stay in the club.
If there are sufficient people unhappy with Elm but are cohesive enough to push the compiler forward, then why not?