That sounds good but in custom battles that would not work so well. One idea I have is that we could make medieval units have better defense if they have armour but make them slower and make gunpowder units cheap due to the low level of training required. Pike units of the shot era could also be made somewhat cheaper due to lack of armour, but i do not think that there should be an overall rise in stats because that would be unrealistic and it would be fun to use units from both ages. Gunpowder weaponry did not render medieval tactics and units obsolete overnight so gunpowder age units should not be better in every way.

Summary of Gunpowder age improvements: Faster, cheaper, more powerful ranged weapons, gunpowder ranged units cause fear in medieval age units if feasible.