Well, technically it COULD happen. Luffy can seemingly pull anything he wants out of his head while in Gear 5. So, it's not impossible that Luffy could literally pull Nika or Joyboy out of his head to team up with all of them.
But, I doubt that's the way it's going to happen. I don't feel like Oda will go that way with it. If only because Luffy really doesn't care about history. So, I doubt he'd even know enough about Nika or Joyboy to even imagine what they'd look like.
He'd be more likely to create a dream-version of Ace to fight alongside him and Sabo. But, then we'd have to watch Ace "die" a second time. So, I'm not sure if Oda would actually do that. Or if he definitely would, for the feels.
But...I do think it's possible that Luffy could at least "speak" to Nika in his mind, via a "awakened" form of the Voice of All Things. Talking to the mind in his own Devil Fruit. That may be how we get the Ancient Kingdom flashback. Even that may be unlikely, since it would feel pretty similar to Naruto talking to his mom and dad.
I don't know. There are ways Oda could play with the idea a little. But, I just have a feeling that Nika and Joyboy are most likely just going to stay in the flashbacks.