The owner of the original has been inactive and I can't have a discussion or get my changes merged, so I will have to add a version to my fork. With the changes being backwards incompatible, according to semver this would be version 2.0 and it feels incredibly wrong to release a major version of someone elses project without having talked to them.
How do others handle this and similar situations?
As for legal obligations, that very much depends on the license used by the original project. With the MIT license for example you're allowed to make changes and release those, but aren't allowed to claim the whole thing as your own creation (which makes sense because it's not). Not sure about changing names but iirc that's okay.
Anyways, you should probably read the license of what you're forking, but in general it should be OK to just fork it and give credit to the person who made the original. Again, though, read the license. Also, it's probably a nice thing to do to contact the original developer and tell them you're forking them.