My suggestions are as follows:
I haven't inked it, (probably should have, in hindsight) so I hope it's readable. And please excuse my doodle writing. It's a habit.
Now, I had these as ideas for actual Soldiers, but I doubt they'll be used as such.
So here's what I hoped:
Senshi Warriors: do normal Knight damage, but have a chance to stun, and generally just look awesome.
Upgrades:
Bushido - When at less than 50% health, their attack speed and damage increase.
Bamboo Armor - Reduce incoming damage by 8%.
Kamekaze - When at 10% health or lower, they will suicide themselves, doing massive damage.
Invictus Legions: Do half of normal Knight damage, but have incredible health and armor.
Upgrades:
Scuta - A large Roman shield, capable of blocking 2 enemies at a time.
Centurion - Better equipped with armor. Capable of taking less damage.
Spear - Romans used to have a row of spear-throwers at the ready in a Legion. Makes Legionaires capable of throwing a spear once a minute.
Clan O'Donnald: No armor at ALL, but high damage and speed.
Upgrades:
Haggis: Like the upgrade 'Well Fed' or whatever it was, it gives them higher health, but on the battlefield instead of upgrade menu.
Highland Ale: Every once and awhile, they will do double damage, with a chance to miss and TAKE double damage for 10 seconds.
Battle Kilts: Makes them even faster, letting them dash into combat without fear of their Kilts exposing their Highland jewels
EDIT: I really liked Norbz idea of Allies. They would be awesome, like your normal Reinforcements, but with different abilities. Lets say it comes with 3 guys, (4 if you get Premium) and they would each have their own set of upgrades, on the field and in the upgrade menu. Maybe the group would consist of an Elf, (fires a bow) a Dwarf, (throws bombs) and a Gnome (pickpockets for gold). With premium, maybe they could throw in Freddo the Hobbit
Failing my ideas, I present the Community with a whole new concept: (I hope) Skins.
Being able to skin, or have pro artists skin the game for you gives it Personalization Value (PV). If personalized, a game can have indefinite appeal, because everything looks so different and neat!
Take a page out of Mojang's book here, and let there be user-made skins. Have there be SOME PV somewhere. It can make a game really attractive.