I have the idea of the tech tree composed of many fields with several properties, something like that:
Properties: price, the necessary resources to production of the one unit, health points (HP), damage threshold (damage subtracted from the power of the weapon), maintenance, weight etc.
You can choose your own name to the armor, like: "light armor I". Then you choose properties to reasearch like price -3% (you can choose how much % you want - it has influence to the amount of the needed research points (RP)).
If you want to upgrade your "light armor I", then you choose properties again, like: weight - 3%, however this time the upgrade is more expansive (proportionally to the RP paid for the previous development of this armor), however...
...You can begin a new project, and name it like "heavy armor" and research to it a new bonuses e.g. HP + 2% etc.
Researching some bonuses may change other properties of the armor, so when you finish HP+10% then it change:
- price + 1%
- needed resources + 2%
- weight + 1%
- maintenance + 3%
This will force players to choose specialization in each field, so each empire will be unique.
In research should be the threshold of the effectivity. Above it any additional money will bring less benefits than below. It will force players to spend all RP to one technology and get it faster or spend RP to many technologies and get them later at the same price.