Here is the list of ideas that might never implemented (and lately if they are) :
Persistent wreckage : We you fight on a map, the wreckages of the previous battle are still there.
Asymmetric fights : ie. one player control a stream of units attacking another player.
Objectives missions : On top of the "classic" escorts missions, you may have solo-like objectives. ie the defending player have to protect some civilians, evacuate them. The attacking player has to take control of some strategic building to have some support.
Stupid fights : Remember the mission in supreme commander 2 when you have to capture a base against a constant stream of engys ?
maps POI : Some building might be "hackable" to take control of neutral defenses on the ground or satellites.
Home systems : When a faction is about to lose, their last stand will be on their home system. Each system will have specific attributes for the faction, making it harder to capture :
ie.
The seraphim planets are really close to a sun, resulting in a constant lose of HP for all units. The ACU/SCU regen fields counter it perfectly.
The cybran system is highly technological, and there is full radar coverage on each planet for every player, making the stealth mandatory.
The UEF system is of course the solar system, and UEF will have highly fortified bases.
The Aeon planet has some special ground making no-hovering units slower. Also, a lot of rivers and lakes.
To have a little chance to be implemented, we need to reach the goal
(If I list these ideas, that's because I know it's possible to do, but not so easy to do .