Personally I think The Iron Bull from Dragon Age Inquisition is a better example of what Aronna could be, if written properly. While Bull joins under mostly peaceful terms, he is a spy for the Qunari. While it never plays out in an interesting way imo, your relationship with Bull for the most part is purely business. At the start at least. Depending on your decisions he can become a lover, brother in arms or betray in the final dlc.
Aronna could be something similar. (Not the spy thing.) But your relationship could be this mostly antagonistic/convenient solution to each others problem. Which is how I always read it. As helping her defeat her brother is done to stop the raid on Hawethorne. Not really because you like her. And she just wants to be Chief/disagrees with some of the things hes doing.
The problem like most people have already pointed out, is what comes after that. If she's chief why would she come with you? If you're her dom you could make the argument she has no real choice. What you say goes. And at that point its up to the player if she joins the party. This could lead to her disagreeing with the things you're asking her to do, or you having to stop her from raping defeated enemies. As thats what orcs or would do. Or maybe your champ does those things also and you two get along perfectly. (This could lead to multiple paths)
Maybe you kick the Orcs out completely and send them back home. Or (which is pretty clear) you're the real chief. And Aronna is keeping the throne warm for you. You literally get to fuck her in front of all the orcs after she announces herself chief. Pretty much the most power fantasy thing these devs let you do in this game.
If you're her sub, personally I think it should lead to a bad end. As why the fuck would she stop raiding Hawethorne. It's champion is a fuckboi/girl. And Aronna is the strongest person in the valley. With the exception of Kas. Personally I don't think their should a sub champion route. I think it should be turned into a power bottom path. It is essentially the same as how DAI made the inquistor that gets romantically involved with The Iron Bull a power Bottom. And its pretty much already implied as if you defeat her, and become her dom, you have the option to challenge her to a wrestling match and lose on purpose, so she becomes the dom. Could lead to some funny dialouge. (In DAI I really liked the dialogue when Bull is in the party with Cole. Cole pretty much spells it out that Bull isn't really the one in charge and that the Inquistor only lets Bull top them because he needs to feel a like a big strong man. While the Inquistor needs a distraction.)
I don't think Aronna is the worst character. I like how she develops for the most part, though I think if you got more dialogue when shes ambushing you, and it lead to actual exploration of her character it would help make the shift more believable. Instead of repeatedly fucking her or sicking Brint on her. She'd be one the games stronger characters. She's a perfect example of how this game has potential to make an interesting characters, but some issues in the writing fail to bring out her better qualities.