It would have been awesome if she was the villain. I enjoyed Just Cause 4 in some areas, but overall, it was a little disappointing. And one reason it was a little disappointing was because it had concepts it didn't take advantage of. There's a lot of cutscene material for Gabriella and Oscar. There were also interesting fights that could have taken place, but nope.
One reason I think she should have been the villain is because she looks like she has more guts than Oscar. Oscar looks like the person to run away from a fight, but Gabriella looks like someone who would run straight into a fight.