They said that the earliest cannon was unearthed around 300AD, which was probably around the late Mythic Age in AOM. Therefore, I believe it is within the time range to add a Cannon unit to AOM. Probably, it could be in the Greek civilisation, as an expensive upgrade from Petrobolos, since Greece was very close to Persia, where that cannon was unearthed. The cannon could fire a cannonball horizontally, destroying almost everything in its path. Can be used against both buildings and units, I suppose, because there is little chance either could stand against a large metal ball roaring at you. However, because of its sheer power, you need to research a very expensive upgrade to turn the petrobolos into the cannon (imagine how much is needed to research explosives). Also, it won't be a bad idea to make it expensive to train each of the cannons, since the iron needed is quite expensive.
Now, with all that planned, we can begin to make it. I know a bit of modeling, but I don't know anything about animating, or importing into the game. I understand I still have a lot to learn in this area. I know some texturing too. I need to find someone who can port it into the game as well as animate it. I could send that person the texture file and model in bmp and 3ds format.