Freeciv has the same "design unit prototype" system as SMAC, all units in freeciv are "unique prototypes" and every civ has their "unique" set of units (that all still look very similar), but set to civ units and simplified, is opensource and the AI likes it:
-Designing units completely by variables.
-The first unit of its type costs extra hammers per civ to limit many different unit types per civ. (actually in freeciv a prototype has to be RESEARCHED like a technology)
-Technologies do NOT give new units but new unit addons and ability slots (sieging, extra movement, extra defense, extra offense, "being ranged", seeing invisible units) to design new units.
-The unit gets its icon/model from the dominating ability and there are some models for mixed addons.
Could Use civ4 starting promotions for a similar system:
-Make special promotions that you can not get by experience but that...
--Are "empty slot promotions", give units up to 3 or 4 "empty slot" promotions.
--Are "filled slot promotions" that replace "empty slots" with abilities such as "offense" "extra movement" "see invisible units"...
--Have to be researched
---research the number of empty slots and ability promotions to fill/replace the slot promotions with.
--Have to have national "prototype buildings" build to enable combinations of different "filled slot" combinations.
---prototype buildings for single "filled slots" such as "defense" or "offense"
---prototype buildings for "filled slot COMBINATIONS, For example a prototype building for the "offense AND movement" combination enables to fill a two (one of them empty) slot unit with both if it already has one (one filled). Adding a prototype building that combines "offense and defense" then still does NOT enable you to promote/upgrade that unit to "offense, defense and movement" , that lacks the "defense and movement" upgrade building.
--Can be bought for gold per unit (upgrading unit) if you have the additional "prototype building" for the upgrade (as above).
-Make ONE unit type for everything (maybe 3: land, sea, air) and set its model completely by its promotions.