Autosaving unnamed files is for some reason a feature not many tools implement. I think I've first seen it in Notepad++ and it just makes so much sense. The only thing I found that works like that without getting into sessions etc is Mousepad. It's also very fast to start. Don't know about highlighting. It probably doesn't exist. I would also give Kate a look. It's more capable than it looks at first and is my current note taking solution with a nested folder structure with md files. (It has recursive folder search and tree view of the folders. All I need)