about the balance of things and hasn't made any Yonko Crew too overpowered, but the addition of those two makes a ton of sense as we progress towards the
War of Thrones.
Mihawk prefers to live in isolation and do whatever he wants, but the isolation has weakened him in a way (even at Marinford he wanted to
measure the gap between himself and Shirohige, instead of being certain of his strength like Shanks when he entered Moby Dick). As for the “do whatever he wants” part, that's the official theme of the Red Hair Pirates, innit — they don't care about the One Piece (Shanks has already seen it with the Roger Pirates), or oppressing people and claiming territories like Big Mom / Kaido — all they ever seem to do is get wasted and lounge around, so there'll be very little pressure to conform — Mihawk could easily do his own thing while Shanks and co. are busy partying it up!
And once the major battle towards the end is over and done with, he can go his own merry way. Significant characters should do significant things, IMO — thus far, Mihawk rekt Don Kreig, didn't go all out at Marineford and kinda trained Zoro — which is a meh output for someone of his stature, so an alliance makes sense for the time being because he can do only so much on his own (in comparison with what he could do with a Yonko in the thick of things).
As regards Marco, he lost his Captain/father at Marineford, his crew after the Payback War, mates like Thatch, Ace, Blackbeard over the years — and cuts a forlorn figure. He too can at least temporary ally with Shanks (like Crocus with Roger after his stint with the Rumbar pirates — coincidentally, Crocus was also a doctor). Plus, Shanks' main objective in terms of the storyline is to make sure that the “rightful heir” Luffy inherits Roger's treasure (that's why he sacrificed an arm when he could have easily used conqueror's haki on the seaking to begin with) — and Marco has ties to Luffy via Ace, so their objectives (for the time being) kinda align. Also, the Red Hair Pirates are likely to clash with Blackbeard at some point — which will give Marco the opportunity to avenge Whitebeard/Ace/Thatch.
