These are roughly the roles that make perfect sense to me at least:
Heavy attack: The obvious cavalry line, starting at Horseman, going through Knight and Cavalry, ending up at Modern Armor. The Tank/Modern Armor penalty against cities should be removed, this line should be the primary offensive unit in the game. All units in the line rely on strategic resources.
Soldier: The obvious line starting at Warrior, continuing through the two Sword units, then into Rifleman and Infantry, and finally Mech Infantry. These guys have no distinct attack bonuses or advantages, but there are no actual counter units either, save for archers/artillery in the tactical sense. The reliance of strategic resources in the early phases of these units is removed later on.
Defense: The line that counters Heavy Attack. Pikemen, Spearmen, Anti-tank Gun. This should be the go-to unit type that ones builds as a tall empire in a war. Good defensive bonuses and promotions, attack bonus against mounted/tanks. Counterable by the Soldier class for cost, so combined arms is a must. Archers and artillery aren't as effective against Defense because of the huge defense bonus and healing. None of these units require any strategic resources, but they suck on the offensive. A Modern unit may be required to make sure the Anti-tank gun doesn't suck against Modern Armor. Something like a Firebase, that is an upgrade option to both the Anti-tank and Anti-air units, covering the roles of both, using the Mobile SAM model and icon, with fluff about anti-tank rockets and anti-air guided missiles.
Patrol/Fast: The roles of this line should be initial scouting and fast anti-harassment response. This should be the Scout>Militia>Musket>Paratrooper>Helicopter line. The Paratrooper really should be late in the Industrial Era, as that is the point where they were most used, culminating in WWII, with their popularity dropping sharply after ~1980., where they get replaced by attack helicopters. All these units are very fast, very mobile, with a good strength, but their strength-to-cost ratio should be nowhere near their Soldier counterparts on the offense, or their Defense counterparts on the defense. They are, however, cheap. And have I mentioned mobile? The helicopter should replace it's anti-tank bonus with a general 20% bonus on the attack, being the only unit in this entire line to require strategic resources.
Ranged: Archers, Chariot Archers, Crossbowmen. Save for the CA, none of these units require strategic resources, but their true upgrade path ends quite early. They continue into either the Soldier line with the Rifleman, or the Patrol line with the Musketman, haven't decided yet. Due to their range, these guys are excellent harassers and counter-harassers in the earliest periods.
Artillery: Starting with the Catapult and ending with the Rocket Artillery, these units suck hardcore vs any other unit in the game unless that unit is fortified. They should have a miserable, pathetic strength, but a huge bonus against cities and fortified units. Their damage vs fortified units should be greater than the ranged equivalents.
Air Superiority: Fighters and Jet Fighters alone make up this line. Not much to say about them. Both use strategic resources which are valuable to industry and other late-game war units, so using these is a pretty big strategic decision.
Air Bombing: Bombers and Stealth Bombers. Bombers should be late Industrial, not just Modern, but their strength should be lowered accordingly. Similar to Artillery, their defensive strength should be miserable. Again, they use strategic resources, bla bla.
Destroyer: I love the way Thal made this line so far.
Battleship: I also love the way Thal made this too.
Submarine: I like how they are in the original game. The first one is a counter unit, and shouldn't ever need any strategic resources, but the nuke sub should use Uranium, not Aluminium.