I'd like to shed a light and possibly discuss the various groups and characters (likely) present at the wedding, and who may be working with whom.
Right now it seems like there're 4 main factions that will be making their move during the wedding: Big Mom and her loyal followers; The Straw Hats and allies; the perpetrators of Jinbe's "mysterious plot", likely Capone and whoever's working with him; and the Vinsmokes caught in the middle. Most of the members of these groups are also already clear: Big Mom has her homies, her children, and the rest of her direct crew; Luffy has the members of the rescue team, Sanji, Jinbe, and possibly Jinbe's crew; and the Vinsmokes will likely only have each other so long as they can't reach their forces.
Of all those, it's only Capone whose real threat level remains unknown, and much of it depends on who's in league with him aside of the probable aid of his crew: Pudding could be making her own move against her mother with his help, or Snack may be thirsting for revenge for whatever Big Mom did to him after his defeat (if he's still alive). And if Jinbe's worried about his plans, he's probably got things planned out all the way.
Of course, Caesar Clown's also still somewhere on the island, and while he already has ties to the straw hats, the last person to possess his heart was Vito, Capone's right hand man. Which begs the question: did Capone hand his heart over to Big Mom, or use it to gain Caesar's aid during his possible Coup? A Logia would be quite an asset in the upcoming 4-way battle royale, even a blackmailed one.
Reijiu, while programmed to obey her father, will likely do what she can to watch over Sanji during the fight. Whether this extends to aiding the straw hats, and how much aid she'll be able to bring will likely depend on her father's orders.
Finally, we still haven't heard or seen anything of Katakuri aside from the name, but it's likely he or she'll be supporting Big Mom. It'd be impressive if Capone had managed to bring in one of the 3 commanders, but that doesn't seem likely.
Add to that an explosive Tamatebako, and it's sure to be one hell of a Tea Party.
That should wrap everything up, but if anyone has further theories about how things will go down, possible alliances or characters i mentioned and possibly forgot to mention, i'm more than happy to discuss.