Since Zoro was the first SH to come into contact with his endgame goal (in Baratie).
It will be more fitting for him to be the last SH to fulfill his dream, Zoro vs Mihawk should be the epilogue of the series, witnessed by all the SH. (again to mirror what happened in Baratie, everyone saw his defeat).
I'm mostly certain that Zoro will have a clash with Shanks, he needs to be acknowledged by Mihawk's rival, and that would be either before shanks's fight with luffy or after that.
So Mihawk joining the RHP is a dead idea to me.
After all things are settled (the final war with th WG), the epic & savage fight between 02 Yonko level beasts shall begin.
& it will be more appropriate for the WSS to get a mini arc, + a well developed flashback, not some low tier backstory (what king got).
Oda will touch on Mihawk's past, personality & what shaped his ambition.
Yeah, Mihawk will be the last top-tier we see in action.