Some designs just don't need to change. The AK should be one of those, I'm pretty sure we'll still be using those in 300 years.
Gun Jesus of Forgotten Weapons actually commented on this. He said that gunpowder-based firearms have reached the apex of their evolution in terms of mechanics - all possible concepts have been explored and all that's left now is mixing and matching the various features, and unless some revolutionary new invention comes along that's pretty much it.
However, what he did also say is that ergonomics is where the focus is now: optics on every rifle, advanced scopes that calculate a firing solution for the user, red dot sights on every pistol, captive pins, springs and much less complex disassembly to make maintenance easier etc.
Also, new trends with ammo, as has recently been popularized by the US military's adoption of a new cartridge specifically designed to combat modern body armor.