Apart from bull pup, a rifles a rifle. The M16 is a great weapon, but an intergrated 5.56mm and 20mm/40mm auto loading grenatde launcher like CIWS or ACIWS seems to be the way to go. However there is no real change in the way the nuts and bolts of the rifle operates. Theres is a reason that the front line russian rifle is a down calibered and slightly modified version of a late 1940's design. Because what was good then is still good now, apart from caliber. I dont think the needs of the infantrymen will change all that much in the forseable future as far as the weapon is concerned, apart from new calibers/ammunition, until man portable directed energy systems such as HEL's are practical. So i dont see any real need for a replacement for the M16, maybe just some modifications if its deemed nesessary. Unless the advantages of the bull pup design (just shorter overall length with the same barrel length) are deemed worth the cost of R&D for a new design, and the cost of an army wide replacement.