@Extra soldiers stats (a'la UFO:Afterlight or whatever).
Why not add these stats as userStat1...10 or whatever, the number should be a multiply of the current stats number, so you could see them through multiplied/switchable soldier's stat windows. The gist of things is that a modder would define what these new stats do, how are they named etc. That'd make for a new entity in the ruleset, like 'items' are now. The 2 properties I can think of now would be 'production' and 'science'. The properties would also include how and if the stat grows. (it'd also mean one could tweak how the original stats work; this also reminds me - there should be an option in any weapon to set what skill is trained by firing it, as in, snap, aim & auto). Naturally even without any properties the new stats could be used to influence weapon's damage, armor regen etc.