Why? Do you really believe it's that far-fetched that RH pirates will be the main ''villains'' of this arc?
The Red Hair Pirates will never be villains. That's why Shamrock was introduced in the first place.
You keep putting "villains" in air-quotes because you actually mean to say "antagonists". And no, I don't expect the RHPs to be antagonists either, not even down the line when the Luffy vs Shanks small clash is bound to happen for Luffy to prove to be Shanks' equal.
The RHPs already left Elbaf; they're not playing a role now. Shamrock already left Elbaf; he isn't playing a role now, either.
The Barto storyline is set up to lead to Shanks questioning Luffy and his allies at a later stage, but it'll be a friendly confrontation.
Shamrock's introduction serves a role for the Mariejois hierarchy and the God Valley backstory. Either Sham will be dealt with in the Final War, or he will be utilized as a stepping stone for BB while also preserving Shanks for another EOS use.
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As for this arc, we already have proper villains and antagonistic forces that may veer more on the protagonists' side.
The former are the HKs. They are in everything "villains", though we're yet to see to what extent of competency. Personally, I don't think more HKs will be summoned to Elbaf. Either Gunko & Co. fall here, and we move on to more interesting local threats in the Heaven Realm; or, after Sommers and Kiringuy go down, Gunko is revealed to be
more than just another HK, potentially a host for IMU.
As for the antagonists who might get in the SHs' way, though they are all on the same side, that's Loki for you (and maybe Mosa). We've gotten glimpses already of Loki being a "good guy". His means, however, are questionable--see threatening the Adam Tree and the wellbeing of the giant villages just to be freed. He and Luffy have the same ultimate goal, but the way Loki goes at it might put him once more in Luffy's way, and this time for more than just a two-punch combo.
It could be the first instance where Luffy turns a Rocks admirer into a Nika follower.
That is, if Loki recovers by Act 3 of this arc.