Each Civ & Era should have slightly varied tech tree with two or three linear branches of unique technologies that become locked in as you research them. Meaning you lock out the other variations of unique technologies depending on what you have chosen on any given gameplay.

For example; American vs German tanks unique to each civ. 

There is also many styles and models of light and heavy tanks for each civilization. Giving 1 option out of 3 possible variations of light tanks, and medium or heavy for any given civ for any given game, ensures each run you can have a new experience.

Such that light tanks with more armour on front/ or back vs more firepower/ or range & mobility in terrain can feel like an entirely new strategy and playthrough.

1. Varied techs and tech tree branches per civ. (unique to civs.)
2. Branches of special techs. (Like archer, swordsmen, tank or fighter plane that become locked in and other variations lock out when researched.)
3.  Unique game experience from new combination of research each game. (Due to varied linear by dynamic branch variations or flow of tech tree's.)