Will never happen. Whenever the US suggests anything that could fix long-standing issues in soccer regarding bad calls, Europeans call us Dumb Yanks, laugh at us, then ignore us.
Then we should do it for MLS. Before I started coaching, I was a ref so I know the LOTG. Basically, the laws state that when in doubt give the benefit of the doubt to the attacking team. So if there's even a question of the whole ball crossing the line, I would (in theory) award it. This is where I think instant replay could come into play. So the ref has awarded a goal, and a natural stoppage has occurred in the game. If you ever watch the clock after a goal, a full minute may elapse before the kickoff happens because of celebrations, protests and what not. Why not take that time to review the goal? Any goal, any offside that results in a goal. The main argument is that reviews will slow down the natural rhythm of the game. BS. There's already a natural stoppage in this case. We already play advantage, and again, in the offside language the officials are to give the benefit of doubt to attacker. So let them play on and bring it back if it's wrong. In the short run, the world will probably call us "dumb yanks" but it's what everyone else is clamoring for.