Since Oda gave us Hanafuda as the Warlord Ace beat. I think soon Oda will tell us how Shanks became a yonko. I think he did the same as Ace. Defeating a former emperor to take their place. I think it will be Shiki and not a new character otherwise we would hear of him.
70% chance it's Wangzhi, and would like to believe he's the OG Yonko along with Kaido, BM, WB
Shiki story is more complicated and was heavily tied with Roger, WB era before 4 Emperors is a thing.
Wangzhi was mentioned recently by BB for a reason i guess......
And it seems he's insanely strong still a top tier pirate that post time skip BB and co need assistance from Marines/Coby to take him down. Dude probably settledown and ruled Hachinosu/Pirate island after losing to Shanks who replaced him as new Emperor of the sea