This might sound sort of harsh, but I kind of can't believe that publishers actually think describing something as "this is a mashup of 'x-popular-thing meets other x-popular-thing'!"will result in that new thing becoming popular, too. I realize they're business people, and therefore they analyze business trends and respond to those trends, but they're sorely mistaken and this is why some new YA books pitched as "THE next big thing! Six figure deal! Woohoo!" aren't selling as much as anticipated.
What readers want are creative, intriguing books that are completely different from anything else they've read before. This requires a risk and businesses hate taking risks more than anything else, aside from going out of business. What they need to do is get more input from people who enjoy YA and read it often rather than going on this "Hunger games meets Twilight!" type of pitch.
When Harry Potter was published, it was the first of its kind. When Twilight was published, it was the first of its kind. When HG was published, it, too, was the first of its kind. They were unique in the realm of YA fiction and they captured audience's attention. Nearly all of the copycats afterwards aside from a select few which were equally creative (Beautiful Creatures, Divergent, ect) sort of floundered.
I'm not the YA guru, but I do read a ton of YA and these are just a few of the things I've noticed.