I really don't like that idea, primarily because it reopens "pledge to another power to hurt their interests" as an option (it's probably easier to swing an alliance vote based on headcount than it is to overcome a small number of dedicated players undermining your systems, especially if you stocked up on free Epic alts)
Secondarily, because there'd be a really strong "RP" tendency to permanently ally all same-superpower powers (regardless of the in-lore differences between their leaderships) against the "common enemy", and at that point Frontier might as well just scrap half of them as superfluous. Extending that, I really don't trust players [1] - defence-focused and risk-averse as they collectively are in other "influence the bubble" mechanisms - not to instantly create a 12-way alliance because every single axis of it has a majority of supporters [2] in both involved powers. After all, there's well over half the bubble going to be not under Power control, and no longer some CC-based reason that no-one wants it, so why bother fighting? It'll take years to capture all of those and reinforce them to Stronghold level before that's necessary.
[1] I'm absolutely fine with other players in my power preferring defence to attack, or wanting to focus on undermining Powers affiliated to other superpowers, to be clear! I have no interest in telling anyone else pledged to my power what to do - it's when they try to stop me undermining Mahon that we'll have a problem.
[2] Probably not the same people for each separate vote.