You could always make the main character a strong willed Akeno. Rias would be dependent on her. Akeno would have to put in effort in order to make full use of her powers, but could also become very strong.
You should slap tHe past you that thought you made a story bad Enough to drop and continue writing this one. it is very hard to find a decent Dxd fanfic with Rais as The main character without issei.
Great story, you have already provided a reason for her to keep the seal on so she desnt destroy the world. Just further develop her ability to access varios powers.