If you've read my review of 300: Rise of an Empire, you'll see that one of my biggest areas of disappointment was the characterization of Eva Green's Artemisia, based on the historical female Persian admiral Artemisia I of Caria. She had the potential to be an awesome character, but the way they handled her not only weakened Xerxes (the primary villain of the first film) and made her too overblown, but also clashed a whole lot with the historical figure she's based on.
Obviously the "300" films are historical fantasy and thus expecting accuracy would be kind of ridiculous. However, in the first film the historical characters, though depicted fantastically, are still the same people. Though he goes into battle in a cape and leather Speedo rather than actual armor, Leonidas is still a king of Sparta raised in the Spartan traditions. And though Xerxes looks ridiculous, he's still the Persian king. And in "Rise of an Empire," Themistocles is still himself.
"300: Rise of an Empire" depicts Artemisia as bearing a deep grudge against Greece due to having her family murdered and herself spending years being raped by Greek sailors as a sex slave aboard a ship. Once the men have gotten bored with her, she's left bloodied and dying on the street, only to be adopted by the disrespectful Persian ambassador (last seen being kicked into a well by King Leonidas in the first film), who trained her to be a warrior.
The historical Artemisia was part of the royal family of a Persian vassal state in Asia Minor. The Persian Wars began with Greece invading Persian territory in support of rebellious Greeks and before they were forced back, the soldiers of Athens and Sparta did thoroughly trash one city. Have Artemisia's family killed in the city's defense and have her carried off as "war booty" (in both senses of the term) by the victorious Greeks, with the Persian ambassador from the first movie negotiating her return after the Greeks' victory at Marathon and serving as regent for her until she comes of age. Horrified by her mistreatment, the ambassador has her trained as a warrior for the remainder of her childhood (like in the film) to ensure she'll always be able to protect herself. She'll recover, marry, and have a son, only for her husband to die, possibly as a result of further shenanigans by the Greeks. When Xerxes invades Greece to avenge his father, Artemisia attaches herself to that and becomes a powerful adviser and commander. Her reduced role in Xerxes' decision to go to war not only makes for a more powerful and realistic Xerxes (who comes off in the film as kind of her puppet) but ties her in more firmly with her historical counterpart in the same vein as Leonidas and Themistocles. This revised back-story makes things a lot more "gray"--the Persians have defensible reasons for war with Greece rather than being a bunch of conquest-minded monsters.
Like real history (and in the actual film), Artemisia commands at Artemisium and Salamis. I'd depict her as being more competent than the film does--in the initial battles the Greeks inflict fairly one-sided spankings on the Persians, in at least one case due to the Persians being both arrogant and inept. The film could have the Themistocles/Artemisia quasi-romantic subplot, albeit not as poorly executed as in the film.
(Although their "hatesex" scene got a lot of praise in some of the reviews I've read, I thought it got ridiculous really fast. If Themistocles is a typically-sexist Athenian who gets roughly handled by Artemisia and is intrigued when she offers to parley, that would further emphasize one of the film's strong points--the humanization of the previously-monstrous Persians--and make Artemisia a "strong female character" in a less over-the-top way.)
However, Artemisia wouldn't be as stupidly arrogant and disrespectful of Xerxes as in the film and would survive to play the role she did historically--she advised Xerxes to pull out of Greece proper and leave the fighting to his general Mardonius, which would tie in with the Battle of Plataea seen at the end of "300." By all means keep her depiction as a leather clad angel ninja of death with stylized skeletal armor, but also depict her cleverness in escaping the disaster at Salamis rather than having her getting her entire command obliterated.
I can imagine I'm going to get criticism for depicting Artemisia as overly tied in with male characters (her late husband, the son for whom she was regent, Xerxes) rather than being as independent as in the film, but there are some good dramatic reasons for this on top of preserving at least some historical accuracy.
*Depicting her leaving her young son to go to war would have the same kind of pathos seeing men doing this that most war films have. Furthermore, given how a substantial percentage of American soldiers serving abroad in the War on Terror are female and have left children behind, this would echo that. It could also show Artemisia in a similar light to Leonidas' mother (seen early in "300" weeping as the child Leonidas is taken from her to be raised in the barracks).
*As I've said before, it gives Xerxes more agency while still leaving her a respectable amount.
*Depicting her having a normal life after being raped and only going to war due to political duty or vengeance for a more immediate wrong unrelated to the rape would drastically counter the "ruined forever" depiction of rape victims common in media. That has its own historical problems given how fixated many ancient (and not so ancient) cultures were on women being virgins before marriage, but it could be explained away that to due to her high social rank (her consort would co-rule a major city), her intelligence, or the sheer uniqueness of her being a female warrior, many men might overlook that.
*Artemisia would survive and retain a position of honor in the Persian Empire, which would further humanize the Persians--they're not cliches whose management style can be summed up in the TVTrope You Have Failed Me. Plus if they make a third movie, she can be present for that. :)