Until something really new pops up, I think it is safe to assume we will keep the M-16/C-7 family of weapons for many years to come. The only possible alternative with current technology is something based on the AK-47 action, which would make a fine weapon almost immune to dirt and environmental factors, but is it worth the cost or buying all new rifles?
The attempts to replace the assault rifle have been going on for years. The SPIW project was launched in the 1960s in an attempt to bring extreme engineering to the high velocity/low mass paradigm of the 5.56 round, eventually settling on tiny darts moving at over 4000 ft/sec. The HK G-11 was an attempt to increase PK by firing at such a high cyclic rate on 3 round burst that the shooter felt the impulse after the last round left the barrel. OCIW is/was a project to put "smart rounds" into the hands of the Infantry soldier. The Australian "Metalstorm" company is currently working on a weapons system where the barrel also serves as the magazine and firing mechanism; current efforts are focused on area weapons, but the inventor had built a SMG sized weapon that could fire at 1,000,000 rpm.
All these ideas have flaws, mostly involving the fact they were not "soldier proof". Maybe some day.....