As we all know, Kaido and Big Mom decided to form an alliance in order to defeat every single pirate and conquer the world. That means Luffy will be facing a greater threat than ever before.
I don’t think Luffy and his teammates can fight two Emperors at the same time. So, I think there will be someone who helps Luffy to bring down Kaido and Big Mom Alliance. Here are multiple possibilities:

1: World Government

As Garp mentioned before, the return of the Rox pirates will be a danger to the World Government. We already know Kaido and Big Mom were both Rox Pirates, so with their massive resources and crews, they could potentially destroy the Marines.
Most notably, Big Mom and Kaido recently decided to form an alliance. This makes me think World Government will do something to bring down their alliance. World Government will indirectly help Luffy, even though that doesn’t make them an ally to Luffy.

2: Blackbeard

Why Blackbeard? With a battle involving the Emperors about descending on Wano, Blackbeard will naturally make a move to bring him closer to becoming the pirate king. However, he doesn’t have much to gain by joining the battle himself unless he was certain he could eliminate one of his rivals by doing so.
So, how can Blackbeard make the most out of this situation?
Naturally, that is by launching an attack on Whole Cake island while Big Mom is gone. I believe this will cause Big Mom to leave Wano, letting the Straw Hats avoid fighting two Emperors at once.

3: Remnants of the Whitebeard Pirates and the Straw Hat Grand Fleet

A lot of people already expects Marco to join the final fight against Kaido in Wano, but I don’t hear much about the others. At this point, Luffy really needs all the help he can get, so there is a chance that Marco is gathering what is left of Whitebeard’s crew including Vista and Jozu.
Along with the Straw Hat Grand Fleet, they will have a fight with Big Mom pirates. This will help Luffy to focus on the fight against Kaido.