Up front, I'm going to tell you that I am not a fan of the single female protagonist. As a lover of anything termed a "guy-read" you can guess that I'd rather read about heroic men then tea sipping ladies, or, even worse, ladies who think they can kick people's buttocks and take over anything the hero might have done. (I of course, have exceptions, as I hold a very very high respect for Louisa May Alcott and her works, as well as her ladies, but none of them are the latter mentioned either.) To me, a female protagonist can really make or break a book, as I'm sure it also can for other people. Today, I'm going to give you writers a little advice on the matter. I don't mean for you to follow it, of course; as with all my advice, it is simply my opinion and you can throw rotten fruit at me if you want to.
A female protagonist really needs to be strong. But strong female characters can come off as "liberated women" A.K.A. Suffragettes. This is something you do not want unless you are writing that kind of thing. If that is that case, just don't expect me to read it. A strong female character usually has a man by her side. The man might even be the real protagonist in the story and she his support and love interest; and when I say that, I don't mean she's just there to look pretty either. She can have her hand in the plot line as much as the hero if you wish it. Do your hero a huge favor and give him a lady who will not get in his way in the last battle, but who can also take care of herself so he does not have to worry about her while the baddie is after him. Again, I'm going to mention John Flanagan's Ranger's Apprentice books. All his girl/hero couples are as described above. And when reading his books, you can see that it works out well. My Kilroy and Sylvia are like this. She knows he knows what he's doing, and he likewise respects that she knows what she's doing. That's the best way to describe it. In an adventure novel, the stronger the female character, the closer to the conflict she gets to be. If she's a pretty, soft-spoken, tea-sipping type of lass, then she doesn't need to be on the front lines with the hero. And she doesn't need to be on the front lines of the plot either.
Of course, you can have the girl as the main character with the lad as her secondary and maybe love interest. My favorite plot line to use for this character set up is: Girl joins the army dressed as a boy. Yes, I know these stories are predictable and maybe overdone, but it really did happen, you know. Remember Deborah Sampson. The reason I like this plot device so much is because you know these girls aren't going to come across as a Suffragette type, unless she's trying to join the army as a girl to make a point. If she's joining dressed as a lad, she obviously doesn't want anyone to know she's a girl, and they usually did it because they wanted to help their country for good reason, or they were running from something. A great book with this plot device is Ann Rinaldi's book Girl in Blue which can be found in my Amazon Astore.
Some genres expect you to have a strong female character, even if she is not the protagonist. Westerns are like that. Anyone who has ever read or watched a western will know that the ladies get to fight back and keep Derringers in their purses or up their sleeves.
Something I would like to bring up is the fact that, funny enough, men seem to be able to write better female characters than women, one of the reasons I also prefer male authors. Why this is, I don't know, but I think it's a testament to the fact that men don't like their women to be stupid and weak. Again, do your poor tormented heroes a favor and give them a good woman!
And of course, if you're writing historical, no one is going to beat you up for keeping it accurate ;) In fact, you'll get better reviews anyway.
Okay, and here's something that can really determine the life or death of a novel. You need to know how many female characters you are going to put into your novel. Sometimes more than one female character clash badly, especially if they are both strong characters. My advice for this is keep one leading lass, (this doesn't apply to secondary characters that are only going to be seen in a couple scenes) let her marry her hero and only bring more female characters in for the other male characters (side-kicks, the hero's friends or whatever) in the second book. After the hero is happily married and the heroine is not feeling protective. The one thing I hate in a book is a cat fight. If this is what you want, go for it. I'm just not a hugh fan. Guys can beat each other up and become friends, but girls just can't do that. They go looking for revenge and thus, get under people's feet and the plot device to boot. Please try to avoid this. Also, men acting like women and fighting over the girl vocally is not a good plot choice either. Guys beat the stuffing out of each other, they don't "just talk"; NO MATTER what the "poor" heroine caught in the middle might think. Again, a huge make or break point for me.
So, to the ladies: let the heroes be, and if that be not in the passion...then know that in the end, if you do your part right, one and all will cry out, "'tis the fashion!"