It wouldn't be too much of a drastic change. Their dynamic was never reliant on her being a teen, only her being younger than him. In The New Batman Animated series she was 22 or something like that, and the Batgirl Year One comic puts her at 18 when she became Batgirl and in later stories in her 20s.
Bruce and her never had a daughter-father relationship, only a mentor one. I really don't know where the whole "She sees him as a father figure" thing came from. She already has a father, his name is James Gordon and that's all she needs.
Plus... stop me if you heard this one before: Young, highschool kid decides to become a superhero using the gadgets provided to him/her by his/her billionaire menthor.
Faaar too similar to Homecoming. In fact it'd basically be Spiderman Homecoming but with a girl. I'd rather see her in college or working, then you can show something audiences haven't seen before that could be really fun.