With the modding tools we seem to be able to create mods like Crafted Maps, Tuning Packs, Generated Maps, and Game Modes.
My question is : Is it possible to create mods adding new units to the game ? Possibly with custom 3D models.
At the moment no you cannot add new assets. You can only use existing ones.
This should be a high priority for the developers to add. When it is possible to add new models there would be so much more possibilities for creating interesting mods. New civs, new buildings and a lot more.