So the disguise ability from Eowyn is, as it stands, very much useless in my opinion, as it changes only the design of Eowyn, leading to her being harder to recognise by human players, as her model is the one of a standard rohirrim. While this ability is very iconic of unmodded game, it doesn't really has any usefulness as it stands.
For this reason, I'd suggest reworking it to make it more interesting and add to the ability a meaningful choice. The design rework could stay, as I love keeping the iconic things from base game. The choice would be between Eeowyn disguised or undisguised, but the disguise effect could be to give her more armour if there are rohirrim around her (this way enemy player or ai would be disincentived from targetting her when rohirrim are around her).
The not disguised version could have a very low leadership (eventually only for heroes ? Because they know the shieldmaiden of Rohan fight alongside them, and most Rohan heroes know her worth and know her directly and personnaly, at least Theoden and Eomer who are her very family), eventually along a lowered armour as she stands out (and gameplay wise, it could balance her a bit as a new leadership effect would probably be deemed quite strong).
let me know what you think of this suggestion / any edit you'd like to see made.
IN FAVOUR :
The necromancer (at least some rework of the ability, not exactly this suggestion as it stands)
OPPOSED :