Duplicating here the thread from ideas section.

It I think we need the unit upgrade tree rework. 

Firstly, spearmen can’t upgrade into anything (except if you pick the Greek or Persians), so if you had to fight against horses in first era,  then the only nations you can peek are those 2, either you have to build all new units and keep spearmen somewhere far from battlefront.
Secondly, the most hard choice comes in IV era. There you have no any upgrades for your long swordsmen and knights. But the only melee unit in this era is anti-cavalry. So 2/3 of all melee units have just to wait far from battle for at least 1 era. And they might have 3 stars.

 So here my proposition:

1) There shall be no units that can upgrade into arquebusiers. So players shall build this powerful units from zero, and it will remove unbalance, as now you can get them from crossbowman. Further, crossbowmen and swordsman shall be able to upgrade into musketeers. As you can’t upgrade all units at once, players will upgrade their old melee units into musketeers.
2) Also, there shall be some cavalry unit in this era (and in ancient era horseman should gain +1 or +2 strength, as now they underpowered due to all unique units.
3) And there shall be some pike-unit in classical era.
4) Finally, I think that disbanded units shall give you some bonuses. I have created thread already about domestic politics, where proposed the idea of nations in the game, so if it was implied, then disbanded units could provide citizens with some additional bonus for industry or food

That is all for now, as for me in V and VI it is okay with upgrade tree, can’t tell something concrete for now.
I love the idea that you can’t upgrade anything into powerful new units, like tanks or arquebusiers.

Good for me, except 4.