Gogeta won't appear. Vegeta doesn't even know how to do the fusion dance. Do you want Broly to kill the failed fusion between those two?
Besides, Vegito is the superior version.
It's all but confirmed. It's pretty much confirmed.
Besides, I'm sure there's gonna be some sort of retcon to the Fusion dance that lets it happen while also allowing the fusion to be stronger than normal. Heck it could just be that Goku fuses with a KO'd Vegeta using the Potara ear rings, which allows his personality to take control as opposed to Vegeta's. Or it's some kinda Whis fuckery that allows it. Maybe he freezes time so they can learn the dance lol.