Han Shot First wrote...
Maybe her appearance in DA3 is intentional? Perhaps they're going for a Brienne of Tarth vibe?
For those unfamiliar with A Song of Ice and Fire, this is more or less how Brienne is described in the books:
Brienne is distinctly unfeminine in appearance, and is considered extremely unattractive by Westerosi standards. She is very tall, muscular, flat-chested and ungainly, with straw-colored hair and broad, coarse features that are covered in freckles. Her teeth are prominent and crooked, and after her capture by the Brave Companions, two of them are knocked out. Her mouth is wide, her lips are swollen, and her nose has been broken more than once. Despite her overall ugliness, Jaime Lannister notices that her large blue eyes are surprisingly pretty and shine with confidence in battle.
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And Brienne from the Game of Thrones television series:

I love Brienne from GoT (Read all the books). But still - opposed to the change in Cassandra. Again my beef is "Changed for the sake of change". In both the animation film and in DA2, Cassandra looked like Cassandra (Related towards eachother). And again, its not about beauty as much as it is drastic change in appearance for no reason in DA:I.
If you look at Cassandras facial structure in the animation, then in DA:2, then in DA:I - you will see that both her jaw, chin, eyes, brow and nose has undergone a change that age could not have caused.
I will still romance Cassandra in DA:I because of her personality, but her face doesent even remotely look like Cassandras, it will feel like romancing someone else with her personality.
Btw the chemistry between Brienne and Jaime lannister is so well written in the books! People not reading them are missing out big time