They need to port Diablo (the original game) to the DS =D
But actually on topic, I think they'll do a Valve. Blizzard won't want to port it but Activision will, as such a third tier Activision team will do the port. The PC version will be the superior version.
While this is possible, I doubt it will happen. Blizzard hasn't outsorced a port to a console since Starcraft. I think they got tired of the shitty releases of their great games. Consequently I can't imagine they would be excited to have their game ported by some third string developer, and Blizzard will have the final say on whether or not it is ported.