I get another idea, for Rohan late game strength.
Give an extra upgrade in the royal camp summoned 10 points power,"banner of Eorl", this can only be available after you get all other upgrades in the camp and Theoden reaches Lv 10.
This would provide a global leadership for all Rohan heroes once purchased, it will strengthen the final ability of all Rohan heroes.
The cool down time of Theoden, Theored,Eowyn(shield maid and no man I am) and Merry's final abilities and three captains' abilities will be shorten a lot;
The final ability of Eomer becomes "fearless charge", for 15 seconds nearby cavalry will not slow down when crushed enemy infantry and will cause nearby enemy flee uncontrollably, I never like his final ability anyway.
The final ability of Hama will summon royal guard permanently, three hordes at most. The Royal guard could switch to mounted, and could use sword or bow when mounted
The final ability of Gamling, "General mobilization", Gamling summon every men possible to Dunharrow on behalf of King Theoden, for 30 seconds, accelerate the recruitment speed of all stables, ranges and farms, Rohan Infantry will be transformed into corresponding calavary once they approach stud farms/stables/Royal Camp. The cool down time will depend on how many Gathering Bells you have, for each Gathering Bells it will recharge 10% faster, 30% the most.
Anyway, I don't think add elite infantry will be suitable for Rohan's style and background in the book, we should strengthen the heroes and cavalry further instead.