We have no plans for EPUB editor yet (I mean editor of existing EPUBs). What I meant is that we might add the EPUB import capability to Atlantis Word Processor in the future. EPUB would become a fully-fledged document format of Atlantis (like RTF, DOC, or DOCX).
There would be no need in what you call "a pretty modern XHTML+CSS renderer". Most modern word processors have more powerful rendering engines than any existing EPUB reader. The present version of Atlantis Word Processor does not have all the features of MS Word. But Atlantis still has plenty of text formatting options unsupported by the existing eBook readers. Most existing EPUB books can be perfectly rendered by the current engine of Atlantis Word Processor. Atlantis would only have to convert "XHTML+CSS" to its internal format. This is the main difficulty because there are quite many incompatibilities between "XHTML+CSS" and the internal document format of Atlantis.
Thanks for your comments, Valloric.