It's been a year now since the last level storyline update so I would expect it to be coming with the Summer and Fall updates we usually see each year.
Looking back at the updates, it took about 2 years for KI to make the Morganthe arc after the end of the Malistaire one, thus we can probably expect the next story arc to start either early next year or later this year. (there is the possibility that they won't follow that, though)
If you did get to the end of the Morganthe Arc, there is defiantly a story after that with this mysterious figure we know as Old Cob. What are his plans for the Spiral? Does he plan to destroy it? If so why did he want to take down Morganthe and not team up with her? Will we get to fight penguins in Polaris? But this fall possibly there could be test realm for the new story
Honestly, W101 may never end. They will probaly always find something new to make. But i also think eventually they need to stop it. It needs to have a good ending, yes. But they don't need to drag it endessly on. But i am interested in old cob. But yeah that is my opinions on the topic.
Ehh... they're KingsIsle, they'll think of SOMETHING. Remember Dragonspyre? Everyone was like "OHHHH No more Wizard101! It's Over!" Well, KI got something up: Morganath. They'll manage to pull something off again