I think we are already in a battle royal: Think about the many mini clashes we had already. They were not big - but they still count as fights. And now with Saturn and Kizaru coming (and a party from BBPs) there should be enough fighing going on soon.
I do not know if the BB-ship actually means that BB himself is arriving there. We do know that Garp is after him. And I think Oda could focus on this battle first, before he lets the man clash with Luffy. If Garp gets beaten or even killed, than Luffy will loose one of the last people from his childhood, that he calls family. That means in return that with killing/injuring Garp, Luffy will have a much deeper reason for wanting to defeat BB. It could also make BB slide more into the deep antagonistic side.
In another post a while ago, I had written that I think Egghead will actually be a Marineford 2.0 .
Because if Kizaru arrives, we need to consider this line from him:
“Guts alone aren’t enough, Straw hat Luffy. It takes more than effort to save someone. You’ve got to have power.” That line was from Kizaru directed at Luffy during the Marineford war. And it is the most important line that ingrained into Luffy’s brain and is still echoing now into the future and into Egghead arc.
To clarify the situation back then: The moment that Luffy had almost reached Ace, his brother, he had been attacked first by vice admiral Momonga and then by vice admiral Dalmatian, before their superior admiral Kizaru stood before Luffy, blocking his way towards Ace and kicking Luffy away.
Important to notice is that the attacks from all three can be seen as representive strength of the marine altogether. Back then Kizaru (and Dalamtian and Momonga) was most importantly reflecting the might of the marine and the WG.
And if we are heading on to the point in time shortly after the incident at the Marine ford, we see Luffy devastated with the death of his brother. He is blaming himself for not being strong enough. Because he is still not over the defeat and his inability/weakness from back then. He could not save Ace, nor could he save his crew from Kuma and Kizaru/Sentoumaru on Sabaody.
Now however, he is getting again the chance to proof that he has gotten stronger and is in fact able to save his friends/Vegapunk as Kizaru draws nearer with the same military might behind him.
And just when you think it could not become stranger: We also have some brother-like figures currently on Egghead: Shaka can technically be seen as an almost exact copy of Lucci (Look at the body posture, eloquent speech, strategical thinking, both are ready to make sacrafices for their goals, Names: Shaka - Buddha [Representing "Good"] and Lucci - Lucifer [representing "Evil"])
Also, the "that person against that person" from Oda was worded as if the match would be some kind of big surprise for us - as if we as reader would not ever have thought of a fight like this. This means that we will probably have two people fighting, who we would not have ever belived to be fighting against. Also, this fight has a backstory, as Oda had said. So seeing that the room a backstory needs is probably going to be a little much for a side-plot, that battle that he talks about will happen on Egghead most probably. Now think of two persons, who are either currently on Egghead or heading there, who you would not ever have dreamed of fighting against each other.
I have my own inklings as to who it is, seeing as there are already hints present.