My own suggestion would be to go to an AK type weapon. With one M-14 per fire team, for long-distance work.
If that is too complicated, then just bring back the M-14.
The M16 probably needs to Move On... But, it's just too bad Stoner and Kalashnikov aren't around any more, now that the Cold War is over. We could just give the two of them a billion-dollar budget and tell them to agree on a solution to all our problems.
MikTim [Mikhail Timofey'vich] Kalashnikov is alive and well, and continues to maintain his position as dean of Russian firearms design engineers as a consultant to the Russian Izmash-avto arms factory at Izhevsk.