it is quite likely that a new rank above Legendary will be implemented in the future, and I suppose the merit requirement for that rank will be very high, like 2500 merits at least.
I honestly doubt the fact that an extra rank could be added to the forum..
Legendary rank to me should be the final rank attainable,and it doesn't come at an easy task,so we do not have the usual high numbers getting up to that rank,considering how many merits one has to earn.
I doubt theymos is actually thinking about this,there could be other tweaks,maybe a badge,wreath,crown etc on people's profiles when they reach such amount(2500 merits), but I think they'll still be legendaries(i can't think of any more suitable name than this)