Even without official mod tools, Dota 2 players have been making all sorts of Warcraft 3-y custom modes and games for Valve's game (which is, of course, the sequel to a WC3 mod). Some recreate old classics like Pudge Wars, Enfo's Survival, or Element Tower Defense, while others are new short weird things. But they're a pain in the rump to make and to play, unofficial hacky things. Good-o, then, that Valve have now released mod tools.
They tools are very early right now, but the idea is that people will be able to simply download custom modes from the Steam Workshop.
Valve dropped the alpha release of Dota 2's Workshop Tools last night. They say:
"This is a developer focused release and currently has high system requirements, but soon everyone will be able to play. While this is still an early release with some instability, we believe it is important to give the community access to these tools as soon as possible ...
"These tools are for you and we're looking forward to seeing where the community takes us in developing content for the expanding Dota universe."
Right now they require a zippy PC with a 64-bit version of Windows and a Direct3D 11 GPU, but they'll mellow with age and become less demanding. The Dota 2 dev wiki has more info.
I would be delighted to see a bit of the weird and wonderful WC3 mod community spirit come over to Dota 2, especially as being free-to-play means everyone will get to play these creations.