For the amount of money Vince would have to shell out to get Okada to even consider such a thing, I doubt he'd keep Okada on the mid card. He'd likely want to feature him as much as possible to get some of his money's worth. But yeah, I don't see Vince ever even wanting to do that.
But the likeliness of this happening is probably as close to zero as it can be. I get several Japanese wrestling magazines and going to WWE has been brought up several times to Okada in these magazines and his responses are just dismissive. But that's part of his character. He is very dismissive. However his comments about WWE actually exhibit a level of disdain for the idea. He exhibits a level of bitterness about his short time in TNA. He feels WWE would be no different than that.
One of the biggest factors, however, is that Okada simply loves Japan. He has mentioned that he likes touring in the US and doing ROH shows, but would not want to spend the amount of time away from Japan that he would have to if he is in the WWE.
I agree with Stopspot, never say never in wrestling. But this is probably about as likely as Eric Bischoff signing Vince McMahon as a jobber at the height of the Monday Night War.