I really like the idea of tagging - goes a loong way towards ComboBoxes 
There’s one thing I am missing, though: after I created a new option, I don’t see a way to edit it (i.e. to fix typos).
Is there a way to achieve that - and if not: what’s the best way to get such a feature?
This feature is not available “out of the box”, and frankly I’m not sure how you could create a usable and intuitive UI to support it. The most straightforward approach I can think of (caveat: I am not a UE designer) would be to make the tags ContentEditable, and then when the user edits one, update the matching <option> element I the underlying HTML <select> and then trigger the change event so the Select2 widget would update itself. But I have no idea whether that would actually work, and you’d still need to indicate to the user that they can edit the tags, and I’m not sure how you’d do that.
The author of the Select2 widget is working on a new release. There’s a discussion topic in this forum to request new features. You might want to post your request there.