Although this thread (and the conversation) is not about "what if" but about which minor faction we want as a major faction and why.
What if a given faction had a major endgame questline, in other words. I'm sorry if the semantics weren't clear, I may have made a conceptual leap in my head that didn't come through in the writing on that one.
Having the Followers and the Kings band together and control the Mojave (with or without the Securitron Army) goes against what those two minor factions believe and represent.
I don't disagree, and it's not a combo I'd have opted for myself, but gaining control of the Securitrons doesn't have to mean controlling Vegas, especially considering that (as you point out) you'd be putting that power in the hands of someone who wanted to see it used least. Not cosigning bureacracy or a pyramidal hierarchy of authority doesn't necessarily mean you balk at patrolled roads, staffed borders, or a fair and efficient peacekeeping force. We know Emily Ortal would be capable of being one of their botforce techs even though she'd rather be doing R&D-- I imagine MOST of the human logistics arm of the bot force would be less than thrilled at the chore. And Boom! We have an ending for Arcade where he gets to see an independent Vegas AND put his Enclave expertise to use in its service in a semi-martial capacity his father would be proud of, but he'd still get to be just as world weary and unsatisfied as in all his other endings
.
As you say though, again, the King doesn't see himself as the leader he is and that's unlikely to change by any endgame. Pacer is the kind to like to flaunt power and would probably step up if King was dead, but he'd be just another two bit hustling gang/warlord-- Benny, but without vision, cunning, style, or restraint. There's no civil leadership there. Any endgame questline involving those two would probably do better to focus on them securing their own interests during the confusion of the battle, dealing with invasion or impending siege on the strip, or evacuating like the Sorrows and the Khans.
I particularly like the idea of restoring and expanding the "when the moon comes over the tower" quest for a major Followers questline -- gathering info on his operations through Yes Man and various open House Industries network backdoors, infiltrating the bowels of the 38 while he's distracted, and installing hardware and software that would give the Followers access to his data and a way of checking his authority -- essentially, installing them as the "board of directors" he said he would never answer to, establishing a grudging symbiosis on both sides so that he has to play nice-ish in order to realize his vision for Vegas (weakening Vegas' position among the powers and undoubtedly pushing his milestone targets back by a dozen decades, give or take)
So all the endings are just "The Courier managed to control New Vegas, instead of NCR or Legion". That is all that the ending says (and it's all that the Courier achieves in the independent ending), the rest is up to the player to decide.
Like you said, there are thousands of ways one can reform Vegas, but nothing in the endings tell you you didn't reformed Vegas the way you wanted to, it only says you reformed it.
I know this is how most of us choose to play things out in our heads, but I think in actuality the fact that they give very specific ending details about most of the factions and people you interact with means that, by process of elimination, there are a broad range of postgame scenarios and situations which are precluded for Vegas and the greater Mojave wasteland.