i'm not entirely sure of the timelines, so forgive me if i'm wrong, but weren't most of the targaryens dead at that point? i mean, by the time lyanna gave birth? rhaegar was dead, his kids were dead, his ex wife and all the other targaryens were dead, and i don't know if dany was already born or not, but i'm wondering if maybe naming him aegon wasn't a last declaration of love for her dead hubby. like, rhaegar was sure of the prince who was promised and thought it might be of his line. with all his line extinguished, it's possible that lyanna gave him back the legacy of aegon in the last moments of her life.

it's as possible as any other theory, anyway.