Hi Rodolfo,
thanks for your comment. It's true and logical that many people requested this feature for the hub.
However editing the 'subject' field is technically difficult to implement due to synchronisation problems between all the potential existing hub in the world. If they have all their own subjects it's going to be an incredible mess for searching.
So we let this issue for a "maybe" later. There is a tracker issue for it: MDL-23217
Note the solution suggested by Martin on this issue: "There is another free text field for custom tags that you can use for more specific tags within your context, and we can possibly look at providing some way to add a custom menu on top of this later."