I'd do something like this (until the time comes that the game can be expanded) either one of 2 things:
1- No level cap but no new maps or mobs. you could grind and gain levels, go back and do quests that you missed or that are working now.
2- Level cap at 70 as it is now but you can still gain exp. it would just accumulate. (you would mouse over the exp bar to see how much you have) then when the level cap is raised. the accumulated exp would be applied. (ex. say you have enough exp to be lvl 74, when you log in after the update, bang- you are lvl 74.)