Increase base HP
This ofc on it’s own would just break the game in it’s current state. So it would be D tier.
But together with other changes like the mentioned takeaway of Supplies this would bring the line to a much healthier state. Atm it’s a glass cannon and as this in this game either oppressive or (way more often) falls apart very quickly. Having a higher health pool could make the unit way more “balanced” and allow to care more about the individual units, get rewarded for taking care of them. Especially in conjecture with something like a Speed increase this could indeed be part of a healthy rework of the line even with just basic stat changes.
So together with other ideas it definitely has an A-Tier potenital. And we see with civs like Vikings and Armenians that the current state of the game allows oing that direction - though ofc especially the Armenians also show the limits and the need for a compensation somewhere else.