Pikeman to Lancer to Anti Tank to Heli Gunship is the most drastic change in upgrades and unit types.
---- Spearman Role 2 movement (can fortify, get bonus from terrain, bonus vs mounted (50%))
There are 3 UU's now for Spearman: And 1 UU that replaces them (Battering Ram, the Huns, different upgrade path though)
-Pictish Warrior (Celts)
-Immortal (Persia)
-Hoplite (Greece)
2 UU For Pikeman:
-Impi (Zulu)
-Landsknecht Social Policy
------- Lancer Era: Mounted unit, no defensive bonus, no fortify, -33% vs city, 4-5 movement
3 UU for Lancer:
-Hakkapeliitta (Sweden)
-Sipahi (Ottoman)
-Winged Hussar (Poland)
------Anti Tanks and Helicopters have no UU and change their roles into Anti-Armor
-----Anti Tank: 2 movement, 100% vs Tanks, can fortify.
-----Helicopter Gunship : 100% vs tanks, 1 movement per tile, hover, cannot capture city, no terrain bonus, 6 movement, still has to embark on water
I think Pikemen should go to musket men, And Lancers should be an upgrade from knights (longsword to musketman is already a similar medieval to renaisance smaller upgrade.
Anti tanks/Gunships should be their own category, with some tweaking to anti tank to maybe make it unable to capture cities and more appealing to use (helicopter hover over lakes/coast please)
Landship-tank-modern armor should be their own category as well when horse-units become obsolete, maybe add some policy that adds some unique bonus to newly armor units on cities with a factory, in check with balance for (Autocracy Lightning Warfare) or kremlin's 50% production boost (Order), just to promote newly built units over just upgraded cavalry.
Or add a new seperate building like a Vehicle Plant, that boosts production of land vehicles (including mechanized infantry and rocket artillery) by 15% if you have a coal tile. Sometimes you can end up with so much coal, and little use for them, especially when not going for Ironclads. I don't wanna make factory themself too overpowered however.
I feel Caravel to Ironclad is a bit odd, but I guess the huge boost in strength makes up for it if you have coal.