The problem with heavies in DUST is that they were trying to compete with Assaults for the player role. By making some subtle changes I can see need for 3 heavy classes in Phoinex.

Class 1) Sentinel - Currently the sentinel is more of a door kicker class, that puts it in-direct contention with medium class assaults. The sentinel needs to be more of a defensive tank.

Loadout -
1 X Heavy Weapon
1 X Light Weapon
1 X Sidearm

While the Loadout may seem a little excessive at first, the trick is to limit PG/CPU such that you can't fit all three on a decent fit. Then you give sentinels a fitting bonus to a new light weapon type, riot shields.

These riot shields can be wielded side by side with your side arm, or deployed as mobile cover for use with your heavy weapon. Pair this with VERY slow movement to make him unsuitable for assault orientated combat and you are good to go.

Class 2) Grenadier - The next class is all about explosions the more the merrier. The grenadier is designed to deal out long range support and have the durability to survive.

Loadout -
1 X Light Weapon
1 X Sidearm
1 X Heavy Grenade (2 uses)
2 X Medium Grenade (3 uses each)

Now the trick is the sidearm slit is designed for use with a grenade launcher, that allows you grenades to be lobbed further than any unit could throw them.

Class 3 - Anti-Vehicle -- The final class needs to be a dedicated AVer. This will force more AV players into less combat efficient suits I order to better achieve balance.

Loadout -
1 X Heavy Weapon
1 X Light Weapon
1 X Sidearm
1 X Equipment
1 X Grenade

The Loadout here is very intensive but the point as an AVer is that you shouldn't need many modules to do your job. The trick is to send people towards a heavy AV weapon and a lock-on light AV weapon. We do this by giving fitting bonuses to AV based items.

You see personally I would make the commando the assault a medium suit 2 light weapons, sidearm and maybe a grenade. No equipment.

Then your logic and combat engineers would be more common if you gave them 1light and 5+equipment or 1 light, 1side arm and 2-3 equipment respectively.