Since we do have one dominant male, why not have one dominant female as well? Asiatic lions have female-run prides, and within African prides, there are dominant females. It'd still be realistic, even moreso than what we have now.
I've got a few ideas that might make her useful, rather than 'just a pretty lioness we can't chase';

Our Queen would be locked to our accounts and be given her own slot. The Queen wouldn't take up a territory slot, and like our Kings, we could only have one Queen per pride. It should cost the same to retire her as it would a King, and she would not run off if too hungry or bored. Yes, you will have to play with and feed your Queen.

Like Kings, we would be able to chose an heir for her when retiring or when she comes of age. We would be able to chose to keep her looks or use those of the new Queen. Thus, we could customize her or her heir and keep that look for as long as we want to.

Unlike other females, the Queen wouldn't go hunting. She would have a different way of gaining stats. The Queen would be in charge of domestic-related issues. Teaching cubs /adolescents to hunt, fending off males/females at our borders, the likes. Each hour, we could chose to have her perform one of these duties and gain stats/exp. The former would also have a small chance of giving cubs stats, along with a slightly higher chance of giving the Queen stats. The Queen would only be able to take up to five cubs/adolescents under her wing at a time.

She would be able to breed as well, but I think a cub with a Queen should get slightly higher stats. Nothing insane like +50 in all stats, but something small, like +1-5 across the boards.

Edit: Like people have mentioned, this should be optional, like with submales. That way people wouldn't have to necessarily get one if they don't want one.