Heyo Marky :) If you want to change stuff about the game, I think the best approach is always to ask oneself what is best for a player. And if you don't know, then it's always helpful to watch videos on youtube from people playing OpenClonk and see what they are annoyed about. And then try to tackle that.

If you just want to change/modernize stuff just for fun or for the learning experience then that's great as well, of course!

I still think the effort would be most fruitfully invested into the movement system (not getting stuck anymore) or into the mod system (Larry, to recruit new people).

And for the "how?": Yes, just use your fork on github.
If you have nothing in there anymore, you could even delete your fork and fork the repository again.

And then, it would still make most sense to create a different branch for your changes. That makes it easy for you to pull any official OC changes later or even cherry-pick some changes into your fork/master and then synchronize fork/master with openclonk/master. Then you can work normally and have a workflow as everyone expects it and as you would have in a "normal" company as well :)

If you want, I can explain that in more detail.

>- not create a "rival" or "competitor" with a different name

Noone would be mad at you if you did some changes and release the game as OpenClonk Marky Version or whatever :D That's exactly what our license is for

>- separate game engine and game content into separate projects (this also has its drawbacks, of course)

Do you have a good reason for that? Because yes, it does have drawbacks and in other projects I always found it to be very annoying.