Pretty minor and simple thing, but just a quick "Are you sure you want to reject _'s application? Yes - No" thing after pressing Decline would be nice.
I've known people who have accidentally turned down applicants, making it more of a hassle for both leadership and the applicant to get them into the faction.
This issue is pretty much non existant on PC, but for mobile players where the buttons are close together and just brushing decline while scrolling/trying to accept can veto the applicant it'd be helpful to have a secondary "are you sure" kind of thing.
Having it togglable on/off would help with factions who get a ton of applicants they reject often.