Well, they said that a good way to be prepared is to complete the yet to be released this month quest. Also they said we "will be playing this content for the following months". Maybe a world event? or some kind of event. One thing is sure tho, we wont know it until they reveal it
Although RS3 Mobile is 7/8 done, the Summer Update is not it.
"Will the Big Summer Update release be the new 'Mobile in Winter 2017'? No, it's about 7/8ths done. We're very confident in that."
It seems to be something more like in my last post (#6) on this thread, it is tied into the Elder Gods and Dragonkin quest series (Desperate Time):
"The 'next big thing' is part two, coming in summer (the 'big summer update'). The aim is to be a bit more strategic/episodic. Have things tying stuff together, characters repeating themes in every piece of content not just quests. We have places we want to go and we're excited about it. We're going to announce it in a fabulous way."