I really wish more systems would adopt the "copyindent" pattern, where indentation is simply copied from the existing line. I've primarily developed in vim for 15 years and have similarly minimal `vimrc`s (no plugins), but some of the first configurations are always:

  set autoindent
  set copyindent
  set preserveindent

This should be compatible with basically any language (other than some esolangs like "whitespace") and any tab/spaces scheme. There's no need to have per-file/project configuration, as long as you're okay with pressing space 4 times instead of tab in the projects that use 4-space indentation.