Drake will make a comeback. I could see a mirror to Garp/Koby through Drake/Sengoku considering Sengoku was said to have taken Drake in like he did Corazon. Also he's still Captain of Sword, he was humbled quite a bit in Wano, but I still have hope for a redemption.
I wouldn't mind a Big Mom return to get beaten by Luffy to make good on his promise to Katakuri and act as a clean win over a (former) Emperor with both starting at full health. Elbaph would've been the best place, so this definitely seems less likely the longer the arc goes on without her, but Pudding had faith she was still alive and the Yamato cover story could lead into it.
Conqueror's haki is rare, but sometimes folk place restrictions and requirements to it that aren't as rigid as the series has made it seem in my opinion. At it's most basic, it's required or given to people that have the disposition of the King, and we've seen a variety of folks with it. I just feel like we may seen certain characters with it in future and I hope it's not claimed out of nowhere. Also, I don't think too many have it, it's 1 in a million and we're at the place where those rare figures would be concentrated. I think Sanji has a decent shot, especially if Gaban has it, but it won't be the end of the world if never have it.
I would still be up for a Shiki return, even after the latest chapters. Though it'd probably be a reserved role or in conjuction with other villains.
I'm cool with the series continuing for plenty more years and I'm not in a rush for the end so long as Oda stays healthy and we get great moments.