It's most probably about a Villain who was defeated by Shanks in the past & made a comeback
We already had a Former Rocks Pirate (Shiki), a Former Marine (Zephyr), an Impel Down Escapee (Bullet) & Mary Geoise Escapee (Tesoro)
The pattern is that they are Villains who abandoned their Former Group/Holding Place & decided to make a Plan themselves
So if it's someone who holds a Grudge against Shanks, then his Former Group or Job was ruined by Shanks himself
It's possibly a Former Cipher-Pol Agent, just like Who's Who
Maybe after Red-Hair Pirates took Gomu Gomu No Mi, WsW was imprisoned but WG sent CP-0 to retrieve it
And they sent their Strongest Agent (Someone who was Legendary among CP-0 similar to how Lucci was among CP-9)
After Shanks defeated this Agent & ruined his Reputation & then WG abandoned him (Similar to how Spandam abandoned CP-9)
He vowed to come back one day, and this explains why he would also fight Luffy after seeing that he has Gomu Gomu No Mi