Races and tech trees already allow for a wide range of new things. In AoW3 the tech tree is a combination of class and 3 specializations, races can be split into player-races and non-player races (dwellings).
As far as other parts of AoW3 that were expanded upon. They had new strategic map structures, new city buildings (separate from the class system), new unit abilities, new dwellings (non-character races), new campaigns/scenarios, new diplomacy mechanics.
I'm not sure any of them except the campaigns/scenarios would be significant enough individually to be separated into small DLCs, but put together they made for great expansion-sized DLC. Also, the new campaigns usually introduced all of the new features of the expansions together which I really enjoyed.