Well, I've got nothing against you thinking there is something weird going on about Shanks. I don't even disagree with the notion that Shanks might actually be an anti-hero. Shanks being Luffy's opponent has always been a possibility. However, it's a different story when you create a bunch of what-if scenarios with extremely unclear evidence to support a completely different idea. That's all I've been questioning.
Is it possible Shanks has ulterior motives? Sure. Is it possible Shanks will be an antagonist? We don't know. We have to assume whatever ulterior motives Shanks has are actually anti-hero-motives to assume Shanks is going to be an antagonist. So this is a 'what-if' scenario in another "what if" scenario. Now, whether Shanks will be a final villain or not is also a different subject. Then you have the whole Mihawk stuff going on. These are all assumptions on top of assumptions. At this point, you can just "assume" Shanks and Mihwak will kill Zoro and Luffy and we have nothing to question the possibility.
I mean, you are reducing what I'm saying as simple assumptions and not thought out deductions I've made based on my observations in the series.
I never said Shanks IS the final antagonist. I've always said "maybe" he is in the end. All I'm explaining to you is that if you follow the logic that Mihawk is Zoro's final opponent (because the series has essentially beat into the reader that Zoro's goal is Mihawk and thus is highly possible), then its hard to see where he fits with any other faction outside of Shanks' crew, because of his (only) known association with the crew and the loss of his shichibukai title. Realize that if it were not for these things, I would not be saying what I'm saying.
He doesn't fit with Blackbeard for the exact reasons you described before (why would Mihawk ally with an evil pirate), and he doesn't fit with the WG side of things because he just was removed from that faction. All of these factions also seem to have swordsmen that "could" potentially fit Zoro's criteria for a fight.
If Mihawk is meant to be relevant in the final plot, it is safe to say he will be involved with someone or some group. Never has Luffy's crew inexplicably fought individual groups of unrelated people. They always fight factions or people related to one another. Pirate crews, WG groups, Marines, etc.
It's also very safe to say that Zoro won't get the last fight in the series (because why would he), which further harms the idea that he's just going to randomly fight Mihawk in some sectioned off EoS event for Zoro.
Again, deductions based on what we know or what have seen before, not just random assumptions formed by my headcanon.