I agree that Ashley might not be the type to pick up a gun (though if that's the case, I find it hilarious that she has no compunctions about throwing an oil lamp at an enemy and burning them to death
). With regards to her age, I think you have to take into account that she's probably lived a very protected life, and I reckon that's why she comes across as younger.
In reply to the original post, I don't think there's any call for Ashley to return at all. Her story is done and her father isn't even president anymore, so it would be a hell of a stretch to bring her back in any capacity.
I think it's the same with Billy. He's on the run, and if he was to return then it would be obvious that Rebecca lied about his death and they'd have to sort that mess out. Sure they could explain it somehow but I feel that it's best to leave Billy as he is.
As for Jake, I would like him to return, especially after that scene we saw of him at the end of 6. There's still more potential for him, and it's an unpopular opinion but I actually quite liked his character. Whether or not it'll happen... I don't know, especially with the direction RE7 took.