Corporations won't have anything to gain from having armies while the nation state still exists in it's über powerful modern condition. The only private forces are and will be mercs, which still work for the gov.
If we're talking, say, fifty years down the line, the modern warfighter will have changed significantly.
Infantry will be equipped with energy or EM weapons, depending on which technology is cheaper. Powered exoskeletons, vision enhancement and squad communication system complete with heads up display. Armour will be much more potent, and likely cover more of the body. Support equipment like ultralight portable shields, active defense systems, and support targeting system will be commonplace. Maybe even jet packs. Every squad will have heavy weapons like airburst cannons/missile launchers/anti-materiel weapons.
Land vehicles will be remotely operated, mostly indestructible to today's conventional weaponry, and carry big time energy weapons, EM cannons, etc.
Support and observation aircraft will be completely robotic. Fighters and CAS will be remotely operated. Their munitions will be even more devastating than today's top of the line gear. All military aircraft will be stealthed to radar and thermal detection. Countermeasures will include advanced ECM and an active defense system. Chaff and IR will likely be discarded. Pilots will all use 360° camera systems for total situational awareness.
Naval forces will be stealthed similar to a better version of the Zumwalt. Missiles will likely be replaced by railgun cannons.
If orbital weapons platforms are legalized or opened up otherwise kinetic weapons will supersede nukes as the deterrent force, and the first orbital warships might be constructed.
Additionally, support robots will create a new roll on the battlefield, handling all logistics and replacing IFVs as the preferred combined arms vehicle. Combat platforms will be slaved to squad leaders and provide mobile cover and fire support.
That's my speculation, anyways.