I won't drone on so long as my last thread cause this one's relatively simple at face value. When you the player enter a battle, appropriate enemy/friendly fleets (friendly/neutral with your current enemy and hostile to you/your allies, and vice-versa) who are in sensor range and/or pursuing any fleet involved and/or not otherwise occupied can join the player's battle with a delay.
For example, I engage a Merchant Convoy while one of their Strike Fleets is just off-screen. They are so and so far away, so it will take them so and so long to reinforce, for argument's sake I have 10 minutes real-time before I'm outgunned.
As another example, I get intercepted and am massively out-gunned by a Pirate fleet. Some Independent bounty-hunters with even more, better ships than the Pirates are 5 minutes away, recognise my plight and take pity on me so they decide to join in. I can then commit to "moving in to engage", to delay the Pirates long enough for the Bounty Hunters to arrive.
For mechanical simplicity, this functionality could end at normal engagements instead of extending to retreat battles too. I imagine that'd be a lot more work to sort out.
Anyway, this would make joining battles the same for AI joining a Player's battle as it is the other way round. It would allow us to do away with the scenario where you're just a pixel too close to a second enemy fleet when engaging the first, or just a pixel far enough to dodge having to deal with it. Instead, if another enemy fleet's in range and not otherwise engaged it will join the battle, it's just a question of whether you'll be done before they do.