I was going to mention this, but you beat me to it lol. I also dont think chopper or ussop really know much about haki cuz if they did, they wouldve realized it during luffys battle with hammond and his men.
As for the rest of the crew i definetley think Robin knows about it, and with that knowledge may have tried to develop some form of it (as i dont think there would be a WHOLE lot more she could do with her df in terms of upgrades, besides bigger appendages lol) as well as improving upon her physical abilities (as rayleigh said, everyone has potential to use it, they just dont know about it. so if she knows, she may try to harness it somehow).
Zoro and Sanji may have experienced it already having fought ppl who may use it, and could have started to develop signs of it which may be more evident in the future. (lol, i notice neither of them are around whenever luffy uses it).
And for Franky yeah it would be cool if he knew haki, but in my opinion if he never learned it he wouldnt be out of the loop or weak or nuthin. He's a cyborg for cryin out loud, anything u can do with haki he could replicate with upgrades to his body! Color of observation: he could get x-ray eyes or eyes behind his head lol. Color of armament: he could get denser/stronger armor plating than wat he already has. Color of conqueror: well lets not get ahead of ourselves lol, he's strong but he wont ever be THAT strong (i.e. shanks, wb level) i mean not even marco has it, and u see how much of a beast he is.
About luffy and his gears, i think there is room for there to be a gear 4th or whatever he'd call it, but asking for a 5th,6th,etc is pushing it. remember how luffy combined 2nd and 3rd to fight Asgard Moria. Well wat if he did that again, proportioned the air in his body to go to all 4 limbs as well as his chest (and his hair, so he could get another afro lol), and then to top it all off added color or armament to vulcanize his body! He'd look like an all black and shiny version of nightmare luffy lol (with a fro') with sum difference here and there. he'd have gear 2 speed, gear 3 strentgh, and this new gear 4 defense and style appeal. *.* (plz look into this oda!)
And about them finding df's, i think it would be darn cool if they did come upon one along their journeys (hell, y not make it more than one) and then they hold onto it and give it to the next potential crew member. Plus, if it were a logia fruit, we would finally fill the logia void in the crew and see that new member grow with their powers (and hopefully they'd already be strong b4 the fruit, hence when they get it they'd have a high growth rate and become a part of the "monster quartet". lol sounds weird but couldnt find another cool way to say four). Well, if not logia i wouldnt mind a mythical zoan either, but just please NO MORE PARAMECIA! Unless its epic like wb or magellans.
Well thats my five cents. Wasnt planning to write out that much, but once i started i just couldnt stop. Sorta like oda lol
P.S. i too think it would be too soon to fight smoker right after stepping into the new world, but if they were to meet maybe someone interferes again, (like how his dad, bro, and potential wifey did lol) like say luffys long lost mother

or his gramps now that he's out the marines.