Well, right now, I'm really focusing on just the futuristic/modern side of things. We'll be working on Traditionalists later down the road. As far as shields go, I am planning to have them be in the game, heres my thinking...
Rifles are more powerful than pistols. More damage, higher accuracy, etc. Unfortunately, they're two handed. No shield. You can use a shield with pistols, so this will add some diversity to ranged combat. When ranking up your troops, you'll have to ask yourself, do I want a rifleman that deals a lot of damage? Or a pistoleer that can more easily protect himself?
I'm not quite sure what I'm going to do about melee troops. We'll see about giving them shields. I'm thinking of making two different melee troop types. Assault troops, and Support troops. Assault troops are heavily armored, probably with a high damage dealing two handed weapon. Support troops have less armor, but have a shield, and have a one handed weapon. This adds quite a bit to tactics. Your support troops can guard your rifleman, but if you can get in assault troops close enough to the enemy, they'll decimate them. Or a combination of both. Have a line of support troops guard the assault troops, and charge your way to the enemy. I'll keep thinking about it, but so far, I think that's what I'll do.