I think, it's gonna be flexible: well depended, where the unit fights. To note, 2/3 of RuA was already professional, and it's little problem to learn more than one individual weapon. If we speek about most often contemporal combats, they can happen in city area or on open area as well, while the insurgency and terrorists - are the most often target. Also, it's rare happen when one needs fire from the automatic gun on distans more then 400 m (the effective range of subsonic SP-5/6). If it's happen, snipers do the job, doesnt they.
So in most circumstanses the effective range of 5.56 NATO and 5.45x39 is exessive. Also the effectiveness of 5.56 as well as 5.45 on those distance >400m against body armor is close to zero. Then the Russian 9x39mm can do all the job of 5.56/5.45 and do it even better. Thus I expect great revolution in infantry weapon after Iraq/Afghanistan, like after AK-47 invention or 5.56 coming. Old calibers just cant satisfy the main players. I expect more inclination in Russia and its proxies towards 9x39, which has proved itself as exellent. Of course this Tungsten carbid cor round much more expensive that any 5.56 NATO, and high tech. equipement is needed for its manufacturing, but the soldier's life is still more valuable. It's even good, while an expensive weapon is less accesseble for terrorists.
I also expect recoil-balanced systems for 9x39mm will appear, so the accuratness of fire will not fall behind 5.56mm.
I still dont see any revolutionary round on the West like 9x39 apropos. The turning to M-14 with 7.62 NATO is certainly regress, while its round is too powerfull for accurate automatic fire. Even the old good Russian 7.62x39 1943y is better for such purpose. So, may be the turn for completely new 6.xx mm expected indeed...