No offense, but its a really really really bad idea.
Immediate effects: less rewards overall.
Older/stronger groups monopolize rewards but can't benefit much from it. To be competitive in something so big, you probably need at least 27 top tier players active, but again..only 9-10 packs to share. As with cross server sage, dont expect any improvements in reward structure.

Eventual effects: Small and moderate groups die > Players go to the only remaining groups in their server > rewards again shared by a much larger pool of people > Death of GNW for certain servers who are already dying.

Yeah i basically echo what everyone above is saying. Another question would be.. What does X-server GNW bring? It doesn't add anything for gameplay and actually removes one of the only things left which isn't cross server. Everything that became cross server, i've stopped playing because its a waste of time. If playing in big scale highly competitive GNW is something you wanted, make a new account in servers which have those.