Well, actually, I think Zwiebelchen did try to do something similar with the release of Starcraft II, and the Hive giving mapping support for those interested in creating custom maps for Starcraft II, although it was a failure or a dead project.
More over, as LoadinG..KzHk said, it will take Gaias to a whole new graphic level. Also gaming level, as this DotA 2 custom map designing will obviously not have the same mechanics as the World Editor has.
I would personally hate to play Gaias on a different plataform besides Warcraft III. This map is actually a Warcraft III custom map. Nothing more, nothing less. But this is just my opinion.