The simple answer is we will introduce new leagues when we introduce those regions, as otherwise there's nowhere for those leagues to take place
Regarding regions, Hoenn has been planned to be introduced to PWO for a long time; and is likely to be the next major project following the completion of the current round of content updates. Plans beyond this have not yet really been discussed. The likely continuation would be Sinnoh, which is the last region which can be accurately mapped with PWO's sprite-based map system. After this things become a lot trickier, as the move to 3D graphics, which actually started in Unova, is not possible to recreate with our resources; and will require a total rethink of how we approach later regions in order to maintain a sense of accuracy.