This is kind of a shot in the dark because realistically I don't expect you Dev's to make what is probably a decently big change just for modders to jump on but, a man can dream!

(This is also something more intended for scripted encounters as I am sure doing this for random encounters would be a huge undertaking and would change how the game plays a good bit.)
Basically I think it would be really cool if the game could handle more sides in combat than just 2 so that I could do something like; you track down a High Value fleet with an experimental ship you want to get your hands on, you engage the enemy and as the battle starts one or two additional fleets jump into the battle from the sides to also attempt to claim the prize. (Just a little example scenario) I could see myself getting alot of use out of this functionality as well as other modders and even could be used in the base game if you think up a good mission idea for it. Watching something like the Hege, Diktat and TT fight over something the player also wants would be very fun indeed.

Just something this gamer/modder would love to see in a future update! Have a great day whoever reads this <3