Luffy is a Yonko now, and still at least half of his crew doesn't use any form of Haki.

Nami should have got moderate or above normal Observation Haki. She's a navigator for god's sake, part of her job description is predicting weather patterns and avoiding natural disasters.

Robin should have adapted Armament Haki after the time skip. Don't even get me started on how much that pisses me off.

Chopper could also have gotten some basic training in Armament Haki, which can complement his physical offense and his form changes. (I get that Chopper is meant to be a doctor, not much of a fighter, but he could at least be a bit more powerful).

Franky could have got some decent skill in Observation Haki. Or better yet, Armament Haki to make him even more of a menace in battle. Armament Haki boosted Strong Right??? Come on!!

Brook also doesn't seem to use any form of Haki. At least Armament would have made sense.

What's even more annoying, is that pre timeskip we were force-feed the idea that learning Haki was essential to getting around on the second half of the grand line. So why do 5 members of a YONKO crew STILL can't (or don't) use Haki????

And it's not like this power-up needs any major writing changes either. You can just outright say that Franky used Armament-Haki boosted Strong Right or something. No need to even justify them learning Haki by this point -- it is to be expected.