Doubtful. With SD and likely Vegas, I doubt the Southwest gets three in a row, and topping out at 32 leaves only one spot. I think Louisville has a good shout for the last spot. Some think Tampa but with the leagues checkered history there and two teams already in FLA, I kinda doubt they'll get it. It should be Detroit but I think some politics and history are gonna prevent it.
Would be really fun for MLS and USL to merge and we get a 4-tier pro/rel thing going. I know the USL added a third tier (League 2) in preparation for possibly starting this up on their own, but I don’t know if the owners are going to vote for it or not.
And just because you institute promotion and relegation doesn’t mean you need to get rid of the MLS Cup either: split the season up and have the top teams fight for the Cup and the bottom teams fight to stay in the top flight (like Liga MX and I think the Belgian league does this too).
PS: I know this would NEVER happen (I mean, I wouldn’t say never, never but it is highly unlikely).