Although I liked one of the four options: two variations for a unit. I don't understand why it needs to be upgraded, as it essentially remains the same unit. The alternative upgrade system in "Tribes of the East" looks much better. The ideal option would be two different units with different abilities but similar characteristics and appearance (essentially, alternative upgrades); for each of them, you could build a dwelling in the city and then recruit them. Moreover, they could be converted into each other! That is, keep the upgraded units as in Heroes 5 (with some changes for Heroes 6) and remove the unnecessary, in my opinion, unupgraded unit. There would be two dwellings, two units, you could have them both in your army at the same time, but each would have its own bonuses, and this would diversify the number of tactical possibilities.
All of this already exists in the fifth installment.