[Nikelaos]

I seem to recall there being a brief period of time when Rohorrim throne was usurped by Wulf of dunland.

You could possibly add more diversity to the nation by setting it during Wulf's reign, making Wulf your unique starting King, and adding Dunlending influences.

Also wasn't Dunharrow used as a refuge by Rohan? (memory fading), if so you've got contact with the oathbreakers (army of the dead) in dunharrow which would allow you to add death magic to the nation. Problem is that the oathbreakers bow only to the heir of isildor...isildor was both gondorian(not rohirrim) and I believe after the time of wulf.

well there's 2 ideas atleast, you may try Wulf's Rohan/Dunland route which since the dunlendings fought for Sauron may allow you to thematically use Rohirrim cavalry alongside mordor forces (pushing it a bit I think though)

or you could go the 'Ally of Gondor' route giving access to the army of the dead (who as it happens were of dunlending heritage), through maybe a hero or unique summon who could freespawn oathbreakers and had some access to death magic. And hell you could even add some gondorian infantry (without making em too good, keep cavalry as the focus but have gondorians for flavour).

I know both ideas are stretching it a bit as the focus is on Rohan, however I'm not sure how else you could balance out the limited magic whilst keeping somewhat thematic and staying true to the lotr representations of rohan.