I think if you tied such new end game areas into npcs in the city hubs on each world as well, you'd go a long way towards making each world feel a bit fuller and more alive as more players would be passing through them.

Can we see some expansion of existing planets, in addition to new worlds? There's no reason why you couldn't theoretically introduce some 'higher-level' areas on certain worlds, and indeed more quest content, even more sandboxy content in additional areas, in order to gradually build towards the feel that other MMOs have of this vast, almost seamless, explorable map.