Well, I have been following this forum for months but I did not fancy writing. Until I saw this thread.
I don't remember when, Team metioned that they don't want to give more than one ability/stance to basic units. So I think Slawek's idea can't be used for basic units at least.
All factions have elite/heroic units which make trouble for opponent. Besides, Gondor and Dwarves have good cavalry and ram riders although they suppose to be infantry based faction. These two factions give a player an oppurtunity to support his army with good cavalry.
Rohan doesn't have proper infantry (except glitched westfold swordmen) for countering enemy pike spam. Team doesn't want to add units for Rohan so player has only two choices: peasant spam or buffed cavalry.
Why Rohan (cavalry faction) must spam peasants to win this game? I don't think most of people are happy while they're playing with Rohan. Spamming weak infantry like Mordor is not lore-wise or fun at all.
I like KryPtik's idea. Purchasable upgrade from armoury (or Spellbook) would improve Rohan's gameplay significantly. As much as i read, there are some inconveniencies for this idea.
1) New buff turn Rohirrim to a warmachine.
This buff can be configurable after testing process. We don't want Rohirrim to wreck all pikes via trampling. However, if cavalry faction can't hold against enemy pike spam why would people want to play with Rohan? If Rohirrim of Eastfold become tankier with this buff, player can support them with Rohirrim Archers. This gives player a chance in LG.
2) Players would not have an option with this buff in spellbook.
Well, I haven't seen any people who choose military camp first. If this upgrade will be purchasable via spellbook, it can be added bottom of military camp. Also players who play with dwarves mostly go for Final Stand. We all know how it is effective with dwarven veterans.
I would like to see it in armoury though. I also support Hamman's Grand Harvest suggestion. It can be purchasable on new Outpost. I think big-village concept will work fine for this upgrade.
My suggestion is about Eomer. He seems like more unit supporter than mass slayer. His piercing spear, memorial and ride of the exiled skills don't fit with his role. Perhaps Team may want to change him a bit. If he is an unit supporter, new buff would be his level 10 passive skill. If he stays as a mass slayer I think his skills should be changed entirely. Rohan heroes except Theoden usually strong in EG an MG. Improvement of Eomer would give option to player in LG. Perhaps his cost can be 1800.
I try to explain my thoughts as much as i can, I hope it can be helpful.