We all know that it is far from leaving a Supreme Commander 3, the last statements of Cris Taylor, is that he is working on a cross-platform RTS, to play on TV, Cellular, X-Box and Computer. Anyway it should not be a Supreme Commander 3.
The current Forged Alliance engine works old but still playable, including excellent FAF work has given life to this game has been abandoned.
My suggestion is to migrate to better engine and we already have it. Supreme Commander 2 uses a superior engine to the Forged Alliance:
https://www.youtube.com/watch?v=jA2epda-RkM&t=4s
Many aspects, maps, explosions, etc ...
This I am suggesting there was already attempt to do more was abandoned:
http://www.moddb.com/mods/scale-mod-fa-economy
Current most playable mod Supreme Commander 2 is Revamp and many units have been withdrawn from the Forged Alliance as every Aeons navy:
http://www.moddb.com/mods/scale-mod-fa-economy/images/aeon-navy-wip-1#imagebox
More mod Revamp is a mix of SupCom 2 + Forged Alliance + New units created by developers. And that's not what you would like to be fulfilled.
The idea or suggestion is to transport the entire Forged Alliance to Supcom 2 by removing units that already exist in Supcom 2.
If you check out a lot of FA things already exist in SC-2 as the adjacency bonus below:
http://www.moddb.com/mods/revamp-mod/images/proxi#imagebox
Scale:
http://www.moddb.com/mods/revamp-mod/images/scale-yer#imagebox
Units T1, T2, T3, yes you have option to upgrade the engineers:
http://www.moddb.com/mods/revamp-mod/images/new-uef-land-units#imagebox
Brick and many others:
http://www.moddb.com/mods/revamp-mod/images/brick-rv#imagebox
So I do not see why we can not have Forged Alliance Forever 2 with the same current game, but only using the new Supreme Commander 2 engine.