It will be great if you don't lock up on US carriers and US numbering plan :)

Also there should be a way to switch off this list of popular carriers, because they may be inapplicable for a particular area in the world.

Also what would be great, is to have a self-service tool for the end-user, so that they can manipulate call forwarding rules by themselves. At best, there would be several profiles, like Available/Unavailable/Vacations, and the user would be able to switch between them quickly with minimal effort.

just my 2 cents :)