If they make a new iMac with the upcoming RDNA 2 GPU's from AMD, and use the new cooling system used for the iMac Pro, the hardware side would be solid.
Then they would have to work really hard and close to developers to bring the most popular and upcoming games to MacOS, or we would still be required to use bootcamp (I don't mind though, but I doubt that would be Apple's strategy).
They have to go a long way to bring AAA-titles to MacOS, but knowing how big Apple is, it's not impossible. Also I don't think this would be very profitable. Maybe in the very long run.
I highly doubt this to happen, but I would be happy enough if they get the hardware part right. I do my work on an iMac, and I play games on them, and I don't want to buy an extra PC just for gaming. I always just buy the iMac with the best GPU, and so far gaming on them is fine, but there's lot of room for improvement.