It would be great to remove restrictions on the tag description field and allow links. One of our publications highly optimises category/tag pages which has worked extremely well, however, due to the limitations on Ghost this is not currently possible.
A workaround is to create a page with the same name as the tag and retrieve its contents into the tag page. If you need help editing your theme to do that, please create a post over in developer help :)
I suppose for a small website this workaround would work but for a website with hundreds of categories this would just create mess.