I suggest adding `--soft` and `--mixed` options to `git checkout`, that act similarly to the corresponding options of `git reset`, i.e. `git checkout --soft <tree-ish>` should move the HEAD to <tree-ish> without affecting the working tree or index, and `git checkout --mixed <tree-ish>` should move the HEAD to <tree-ish> and update the index to match it without changing the working tree.
The difference between this and `git reset` of course would be that, unlike the latter, this doesn't 'drag' the current branch along with HEAD; instead the usual behaviour would apply depending on what exactly <tree-ish> is, i.e. `git checkout [--soft|--mixed] <commit>` would detach HEAD and point it to <commit>, whereas `git checkout [--soft|--mixed] <branch>` would move HEAD and switch from the current branch to <branch>.
I'm aware work arounds exist for these, something like:
```
git checkout --detach
git reset [--soft|--mixed] [<branch>|<commit>]
git checkout [<branch>|<commit>]
```so the aim here is really one of convenience by having this feature contained in a native option.
Another option that might be worth adding to `git checkout` is `--keep-index` (like the option in `git stash`), to move the HEAD to <tree-ish> and update the working tree to match it without changing the index.