One thing to consider, If there will be a new boss associated with the new Mistlands content. If there is a new boss, then venturing to the current Mistlands would be fine. The reason being, A new boss will require a new stone at the starting location and as others have stated previously explored areas do not receive updated content. Therefore old starting area won't get new boss stone.

TLDR; Once Mistlands drops we'll likely have to start new worlds anyway.