e.g. a flat 1/2/3/4 strength (depending on wheat consumption) to offense, infantry defense and cavalry defense stat to each troop per upgrade. This would greatly buff weak units, and slightly buff stronger units (even Imperians).
Or, if that is too overboard, a mix.
e.g. flat 0.5/1/1.5/2 point(s), plus 0.5% base strength per level (level 20 = flat 10/20/30/40 +10% base).
I don't have time to do the maths for each unit using these two methods right now, but I may do so when I come back later and work out which units will benefit and which won't (or if someone else wants to do it out of boredom while I'm gone?). Right now each unit receives a flat 1.5% upgrade per level, maxing at +30% strength at level 20, so there's that to go by.