How about:

having a list of all existing tags in the Edit-Formular as a selectionbox?

Assume I have tagbase=/tags/ and for every tag I have given to articles an existing page there.

Would it be possible to list all these tags together with the Formular, as selectionbox. Maybe even with parsing of the content and preselecting the tags, that are given in the article and vice-versa when selecting the fields then also generating the [[!tag]]-sourcecode ?

this would need a bit JS-work and somehow on compiletime we need to put the list of tags somewhere, where the cgi could read them from. This way, even a pagespec would suffice to determine the usable list of tags and not only the tagbase-variable.

I think this would be very hard to achieve with the current tag plugin, due to the nature of its implementation.

I've had a "tag2" plugin on the go for a while which supports this. It's in a very rough stage but I'll try to find it and upload it somewhere. -- Jon