Take this with a grain of salt. Somebody on the Music Midtown subreddit said that they heard that Bruno Mars, Mumford & Sons, and Blink-182 were headlining MM. In terms of Landmark, Bruno is out with his two shows at the Verizon Center in September, I can't see Blink-182 headlining, and Mumford is a serious possibility. I think Mumford will headline Landmark with Kendrick filling in the rap headlining slot.
if we don't got Kendrick there's always Chance and everybody's favorite J Cole.