Simple - Ability to See inactivity
I think it’s a no-brainer tbh. You don’t want to be in a guild filled with inactive members but especially when people live in different time zones at present you can’t really find out who’s active or not. In my guild there’s some members who play in what would be the afternoon (GMT) and some who play in early morning (GMT). They dont come into much contact with each other in game because of it and it can be especially problematic if guild leaders wish to remove inactive members as they just don’t know who is resulting in either active members who play at different times being kicked or the roster gets long and laborious to navigate.
It should have been a simple feature present from launch. Anet do have the data to determine when a player was last online and i don’t think it’d take that much work to provide it in the guild roster