I hope Arthur makes some sort of faction for humans and somehow in the future stops the Evil Pieces system. And if you will be adding devil's, I'd say it'd be better to focus more on Sona Instead of Rias with Arthur.
Still waiting for that Aoko fight.....
( Hope she remains alive at least, even if she turns into a servant of the protagonist. ) A True Magician as a servant would be pretty cool.
At least spare the Aozaki Sisters......
( Don't take it TOO seriously. While I do want to see them live, even if it's under the Protagonist's control as his servant, I wouldn't want to interfere with your plot ideas. )