I have used Xopus before pretty extensively and I wasn't very impressed (Actually one of the reasons we wrote our own from scratch). You might want to checkout these guys: http://www.fontoxml.com/
Although both of these are commercial products.
I am currently working on the next version of our WYSIWYG (more appropriately WYSIOO) editor for our product easyDITA which runs on eXist (http://easydita.com
). Having written one editor from scratch already, and starting on writing a second version from scratch, I can tell you it's a huge undertaking with lots of gotchas.
It's a possibility that we might release this next version in some way as a community edition which can be used for free for some types of institutions. If you want to chat about it more feel free to message me.