It's possible now to add new civs to AOM. Good luck!

http://aom.heavengames.com/cgi-bin/forums/display.cgi?action=st&fn=19&tn=29942&f=19,29942,0,365&st=0

New civs:

• Aztecs
• Celts
• Babylonians
• Indians
• Aboriginals

New maps:

• Yucatan peninsula
• British islands
• Tiger river
• Deccan
• Australia

New campaigns:

• I don't know yet

A similar project started for AOE1, because Upatch HD made it possible to add new civs:

https://www.reddit.com/r/aoe/comments/7v7q4b/age_of_empires_1_beyond_the_indus_unofficial_fan/