Whatever they substitute will probably be worse.
There are some healthy oils, but they are unlikely candidates. Cod liver oil would put most people off. Olive oil is too expensive. Butter is politically incorrect. So we'll probably end up with some synthesized oil thrown together in an oil refinery, and who knows what that will do to people's health in the long run?
It's not going to matter to me. I do my own cooking these days; I even use the politically incorrect butter. If McDonalds wants to fry its food in some god-knows-what oil in order to appease the fanatics, let them.
By the way, anyone interested in cooking should check out the show "Good Eats" on the food channel. Entertaining AND informative.