Imo Shanks fought Big Mom to become a Yonko. That would explain why shanks is a hero in Elbaf, why Big Mom has a grudge against Shanks, and why he attained the title
No way, if Big Mom was defeated by Shanks she wouldn't be a Yonko and what's more, she wouldn't have a bigger bounty than him.
When you consider all the big shots from Roger's era, the only one that makes sense is Shiki.
- WB, Kaido and BM have been implied to have been undefeated since *at least* Roger's death. And no, I don't think any of Kaido's numbered defeats happened after taking over Wano.
- We know Shanks didn't defeat Mihawk, otherwise he wouldn't be WSS, even though we don't know the full nature of their duels.
- Shanks' attitude and relation with the WG basically deconfirms any high ranking Marine. Shanks is not the type of guy to target the Fleet Admiral or something, he values the balance of the World and he meets with the fucking Elders.
- For that same reasoning I doubt that he has faced against the God Knights' because they're usually recluded on Mariejois.
So all that's left are ex-Rocks Pirates other than the Yonko and Whang-Zi/Ochoku (who was defeated during the timeskip):
- Shiki
- Silver Axe
- Captain John
- Buckin Stussy
- Gloriosa
I doubt Oda would have Shanks brutally defeat an old woman, and among the other 3, Shiki is by far the option that would give Shanks the best portrayal.
Also Shanks' animal motif is a Lion so you have an easy Red-Haired Lion vs Golden Lion theme going on.
I'll repeat, I'm willing to bet on this.