Having a pair of benevolent god-monarchs as the ruling powers in Equestria has worked for thousands of years, but there are some ponies, particularly those sitting one step from the top of the ladder, who feel that having the top of the ladder permanently occupied by all-powerful and, more annoyingly, immortal rulers is violating some unwritten rule somewhere.

And there are always those on the bottom who figure that a little social reshuffle might be in order.

Cover art by the wonderful "sophicabra" on deviantart; I can recommend a gander: http://sophiecabra.deviantart.com/