This^I don't think the AR-15 buying craze is going anywhere anytime soon, but generally I think the next new gun will probably whatever gets issued to the troops as an improvement to the M4/M16 series rifles. Whether it's the SCAR or whatever, the next big thing has always been and will always be whatever the troops are using and in that caliber.
Specifically, I wouldn't be surprised if the next big thing isn't the SCAR or something similar in some little hybrid caliber that's bigger than a 223 and lighter than a 308.
I don't think the AR-15 craze is going anywhere anytime soon, but I would have to another very poplular long gun is the springfield M1A1. just a thought