Personally if their problem is how to implement with the most likely to satisfy the masses, I would say to start with filling in the eyebrow where the painted eyebrow was.
People who play as male Highlander already accepted how the theoretical eyebrow looks like when they choose to fill it with painted eyebrow.
People who don't like male Highlander would not be bothered anyway with or without eyebrows.
It is indeed hard to gauge for people who are repulsed by the lack of eyebrows on Highlander, but atleast the developer can start from somewhere and tweak from that.