In reply to Giorgos Sarigiannidis.

That already exists. It's called LyX. Pair it with LyXBlogger, and you have the best way I know of writing posts for WordPress. (It uses xml-rpc but, since it's all open-source, there's nothing to stop anyone making it work with the REST API as well.)

You even get the added bonus that LyX produces the best PDFs around (and can export to ePub, Markdown, etc) so you can just write in the one program irrespective of the type of output you want.