A most controversial interview. I believe it's time to revisit and see what exactly Oda was talking about here.
Let's take a good look at the interview first:
I think when the interview first came out many assumed the "lurking legend" was referring to a person, but upon rereading the interview at present day, I don't believe Oda was referring to a person at all. In fact when the term "legend" was used, it was used in the same manner that Chinjao used with luffy when he said, "Can you surpass Roger's legend?!" aka Can you create a bigger story than Roger. Then of course we can take a look at Whitebeard saying "many consider it a legend" when talking to shanks, the term legend here not referring to any individual but the events/story/period of time.
This legend was revealed in 957, when Sakazuki states "stop talking like they're some kind of legend", referring to the Rocks pirates.
Then we have Sengoku mentioning that the knowledge about the Rocks is limited to only veteran marines. So it's a legend that not many people know about, lurking in the OP world.
This is also backed by the interview, as it reads on to say "[the legend] will be something related to Whitebeard". Whitebeard being a key figure in the Rocks.
So the lurking legend that Oda wanted to introduce wasn't an actual person, but it was the legend of the Rocks pirates.
Greatest Enemy Ever:
This should go without saying, as the alliance between Kaido & Big Mom is the biggest pirate threat in history of one piece. So Big Mom & Kaido, part of the legend of the Rocks, are the greatest enemy ever for the strawhats. Considering Oda made sure to hype up their threat in chapter 907 with Garp, and then hype up their threat again in 957.
Make Marineford look cute:
I believe this statement refers to the scale & the actual brutality of the war. It is only natural that this would make Marineford look cute as the MF war was essentially a 1sided war for the marines so there isn't a lot of damage done. This one on the other hand will not be the same.
Instead of 1 emperor, we now have two emperors.
Instead of some impel down prisoners, we now have nearly all of the worst generation + strong samurai from Wano + potentially WB pirates coming in (speculative).
Instead of being reserved and being on the defensive, the Marines will now be on the offensive, so no more holding back. And the potential debut of whatever the SSG is or created, that replaced a top tier + other really strong individuals. Seeing as sakazuki isn't a dumbass who'll just sit back while big things happen, I'm sure he'll be acting on what Sengoku stated here.
Lurking legend: Legend (story) of the Rocks
Greatest Enemy Ever: Big mom & Kaido alliance
Make Marineford look cute: Wano War
Anyways, this is my analysis of the interview now at this very moment with the information we have available.
Let's take a good look at the interview first:

I think when the interview first came out many assumed the "lurking legend" was referring to a person, but upon rereading the interview at present day, I don't believe Oda was referring to a person at all. In fact when the term "legend" was used, it was used in the same manner that Chinjao used with luffy when he said, "Can you surpass Roger's legend?!" aka Can you create a bigger story than Roger. Then of course we can take a look at Whitebeard saying "many consider it a legend" when talking to shanks, the term legend here not referring to any individual but the events/story/period of time.

This legend was revealed in 957, when Sakazuki states "stop talking like they're some kind of legend", referring to the Rocks pirates.

Then we have Sengoku mentioning that the knowledge about the Rocks is limited to only veteran marines. So it's a legend that not many people know about, lurking in the OP world.

This is also backed by the interview, as it reads on to say "[the legend] will be something related to Whitebeard". Whitebeard being a key figure in the Rocks.
So the lurking legend that Oda wanted to introduce wasn't an actual person, but it was the legend of the Rocks pirates.
Greatest Enemy Ever:
This should go without saying, as the alliance between Kaido & Big Mom is the biggest pirate threat in history of one piece. So Big Mom & Kaido, part of the legend of the Rocks, are the greatest enemy ever for the strawhats. Considering Oda made sure to hype up their threat in chapter 907 with Garp, and then hype up their threat again in 957.

Make Marineford look cute:
I believe this statement refers to the scale & the actual brutality of the war. It is only natural that this would make Marineford look cute as the MF war was essentially a 1sided war for the marines so there isn't a lot of damage done. This one on the other hand will not be the same.
Instead of 1 emperor, we now have two emperors.
Instead of some impel down prisoners, we now have nearly all of the worst generation + strong samurai from Wano + potentially WB pirates coming in (speculative).
Instead of being reserved and being on the defensive, the Marines will now be on the offensive, so no more holding back. And the potential debut of whatever the SSG is or created, that replaced a top tier + other really strong individuals. Seeing as sakazuki isn't a dumbass who'll just sit back while big things happen, I'm sure he'll be acting on what Sengoku stated here.

Lurking legend: Legend (story) of the Rocks
Greatest Enemy Ever: Big mom & Kaido alliance
Make Marineford look cute: Wano War
Anyways, this is my analysis of the interview now at this very moment with the information we have available.