I looked into creating a ARPG mod for 0 A.D. before (actually a hybrid ARPG/RTS assymetric multiplayer kind of thing), but the problem with the 0 A.D. engine is that compared to more mod oriented engines like quake, the game code and settings etc is too tightly coupled to the main game. This makes it difficult to start prototyping something other that additional factions or other such kind of mods that stay close to the core gameplay. Anything that is a more radical change, like a Moba would be, requires an engine fork in my rather unqualified opinion.
There has been some talk on the 0A.D. boards about a restructuring of the engine to be more total conversion friendly, but so far it has not been done to my knowledge and is also not actively worked on.
“You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete” - Buckminster Fuller