I propose a change in thinking here:
Instead of classifying units into lines considering what they used (Spearman -> Pikeman, Archer -> Crossbowman) perhaps these units should be classified into how they fought? Sounds like a no-brainer, I know, but I don't see it happening here. One thing that I honestly can't stand in Civ3 (RoC included) is how heavily you have to rely on shock units. (Horsemen, Knights, Cavalry, Tanks, Modern Armor.) This really sickens me - where is the offensive infantry? And, in that manner, why is everything so tangled up?
Here is my idea of how the lines should look:
Offensive Infantry - Expensive to build but generally all purpose infantry, traditionally well trained and prepared for invasions abroad.
Spearman (3.2.1) -> Men-at-Arms (3.3.1) -> Pikeman (4.3.1) -> Fusilier (6.4.1) -> Mobile Infantry (8.6.2) -> Mechanized Infantry (16.12.2)
Defensive Infantry - Of average cost and training, generally used as standing armies or as guards for defenseless units.
Archer (1.3.1) -> Crossbowmen (1.4.1) -> Musketmen (2.5.1) -> Riflemen (4.6.1) -> Infantry (6.10.1) -> Modern Infantry (8.14.2)
Guerilla Warriors - Cheap, ragtag armies that rarely stand up to the heat but on occasion come in use.
Warrior (1.1.1) -> Swordsmen (2.2.1) -> Partisan (2.3.1) -> Militia (2.4.1) -> Guerilla (5.6.1) -> TOW Infantry (8.10.1)
Shock Troops - Expensive, fast, and packing a punch. The use of these units should be the exception, used only in extraordinary conditions I.E. when you have high production and low unit support and only need the best, when you need to operate a far reaching strike, or as units to take on the toughest defenders in a city since they can retreat. All have blitz as well; they are shock troops, after all.
Horsemen (3.1.2) -> Knight (4.3.2) -> Cavalry (6.3.3)
(nothing) -> Tank (14.8.2) -> MBT (18.12.3)
Offshore Invaders - Expensive and generally weak, but being able to attack from the sea is a great advantage.
Raider (2.1.1) -> Amphibious Army (3.2.1) -> Colonial Marine (5.3.1) -> Marine (12.8.1)
Explanations for unclear units:
Men-at-Arms - Meant to represent the well organized late antiquity and early medieval armies. Really bad name, needs a better one. The Legion would replace this. Available with a tech off of Warrior Code and Iron Working called Military Organization or some such.
Pikemen - Meant to represent the armies from between the age of the Knight and the age of the Musket. Renaissance armies, essentially.
Mobile, Mechanized and Modern Infantries - Self explanatory, just wanted to point out that the second movement point is due to armored trucks that transport these troops. Oh, and Mobile Infantry is available at Mass Production.
Amphibious Army - Really really really bad name; I couldn't think up a good one. Meant to represent the armies used during the Punic wars, which is where amphibious warfare first developed. Available with the previously mentioned Military Organization tech.
The only major problem I can see with this is which line should be conscripted. I can see real life examples of all three - think of the barbaric armies of swordsmen from the Germanic tribes, think of the massive conscription machine that Napoleon relied upon for his fusiliers, and think of how conscription was in WWI and WWII. I suppose it fits the Defensive Infantry line the best, though, because not all conscription armies were rag tag, and Napoleon's conscription machine was definitely the exception.
Maybe later I'll flesh this out with costs, defensive bombardment, and some graphs.