johto was never released because at the beginning pokemmo had the engine to support the 3rd generation pokemon (fire red pokemon) so Emerald was later implemented (baipassando johto). Later it was decided to go ahead with the regions numerically.   now we come to "when?" some time ago the developers announced that jotho would be added as a region but did not specify a date or time. I believe that the next region will be Jotho because the pokemon generations after the 5th