I think it would be nice to see a DirectX renderer for horde3D but, should it be DirectX 10 or DirectX 9? This is an important question since DirectX 10 will surely be more used in the near future but a lot of people still have non-compatible card.
In the other hand, OpenGL is here to stay. Source engine and Steam is being ported to Linux and some says that Linux will become à gaming platform, check here:
http://www.phoronix.com/scan.php?page=a ... inux&num=1( phoronix announced that ATI drivers was becoming open source before the official announcement. )
If Linux gaming is becoming stronger, this mean that OpenGL will surely be more used for game development, and more supported. I don't worry about the future of OpenGL, and I don't think that a DirectX renderer would be that necessary for PC development. Maybe it's better to concentrate our efforts into a single renderer to be the best as possible? Ive used Ogre, and it had some compatibility issues between the openGL renderer and the DirectX one...