A Karn planeswalker card would be fucking awesome, and I agree that it will probably be made soon. From what they've shown us so far, it's looking to me like Karn is going to be the new Yawgmoth. Only better. Yawgmoth's Phyrexia was composed of machines and undead, while New Phyrexia utilises all the colors.
I think one of two things will happen, if not next then very soon; Dominaria will again face a Phyrexian invasion, or more likely, they'll first focus on one of the other planes, or a whole new one. I think, though that Wizards will hold off on a Dominarian invasion for a few years or so, and give time for New Phyrexia to develope more.