Sub-Commanders

[Tellaris]

Currently, there basically isn't any way to bring in a "sub commander" into a GW battle that you also don't already control. (Via the gate)
What I would like to see is the ability for players to bring along a sub commander to assist them in the fight while not under your control. (Like in the campaign missions, such as Rhiza, Fletcher, Dostya, ect...)
Basically an AI commander that joins you in your next fight, with some restrictions.
You have to pay for it. (It should be really expensive for this reason.)
It takes up a reinforcement slot. (You cannot have any other reinforcements in that slot besides the sub.)
It has a cooldown. (Say 5 min after the start?)
Wherever its spawned in the beacon becomes the AI's "main base" for flagging purposes. (Dunno if this is dynamically even possible.)
Can use Sorian AI Turtle for most maps. (Rush for anything 10x10 or smaller.) (Personally I'd prefer at least the Sorian AI, as the base game AI is much worse.)

Now actually doing this is a big risk. For the Sub Commander to actually be even remotely useless, it needs mass. This is less mass for you to use yourself. Secondly, if it dies, you lose the sub. Third, its AI.
Advantages? Some one else to help you pressure enemy, particularly late games or small maps.
If you're fighting 2v1, it lets you call in some one else for your enemies to chew on for a bit.

As for lore wise, new commanders where subbed to more experienced commanders in SupCom 1, so it fits the theme just fine. (People the player subs to are Crusader Rhiza (1 mission), Colonel Zachery Arnold (3 missions), and Elite Commander Dostya. (1 mission))
It may turn out to be a lot of work for little gain, but that is not for me to judge. I think it would be a neat feature if added.

[Hawkei]

Well as ZeP has stated before. There are no plans for any further features to be implemented at this stage in GW. However, that being said, I like your idea. I have made similar suggestions myself. There are some alternatives/variations on this idea which I outline below:

1. Include SCU as a reinforcement option. This is by far the easiest inclusion to make in GW. The SCU would be purchased, assigned an RF slot, and deployed in precisely the same manner as any other RF unit. With it's own cool down and cost values. It would function as a player controlled unit, but, it would also allow for some interesting tech combinations. Consider the scenario where one faction has enslaved another. A player in the dominant faction could choose to deploy an SCU of the enslaved faction. Thereby giving them a complete dual tech army.

2. Have the SCU assist it's owner on missions as a one off deployed army. Controlled by an AI. (As you are suggesting).

3. Have the SCU occupy an army slot and assist it's owner as a persistent character. This SCU can itself be assassinated. So loosing it in game, means it is gone.

4. Same as option 3. But make the SCU AI controlled and capable of independent operations. This would be useful for consolidating areas which can't be counter-attacked. Avoiding the issue which GW previously had. Where players were forced to "AFK Charge" planets just to get them to 100%. Even though there was no chance of counter-attack. Now, you can tell your AI to do this job instead and log off... If an opponent does manage to defend, they will enter into a game against the AI. If the AI looses the SCU is dies.

[Gorton]

There is now a newer option, because of the efforts of Sheeo and Eximius. They've successfully made a mod allows shared control of armies. I think this would make it possible for a player to join and control a custom, re-balanced sACU (so it's not incredibly OP), and join in and help. Very lore friendly and could make some interesting play.