What about Dragonbinder? The Dragon horn?
http://awoiaf.westeros.org/index.php/Dragon_hornAlso what about this giant shit show!?
http://awoiaf.westeros.org/index.php/Second_siege_of_MeereenI feel like this could all still happen, but altered due to the show already changing major points in it. For example Barristan is already dead, Victarion has yet to be introduced, no mention of a dragon horn is ever really hinted, Hizdahr zo Loraq (Dany's slaver husband) is dead, and Dany is off somewhere fuggin around in the desert with some nomads. However I still think there is some possibility to this happening.
Since there is a temporary truce between Meereen and the rest of the slave cities at the moment, yet their is somewhat of an alliance between the other slave cities still, so they will/have been building up their forces to assault the city, and use the truce as a bait. Also I'm not sure if the Slavers know what is happening with Dany or where she is for that, but maybe if they learn of her missing, they will see that as an opportunity to strike. So as far as the actual battle itself, not really sure who would lead that again, maybe Tyrion again like Blackwater? I would say it would be much smaller forces too, mainly just the Unsullied vs the larger force, yet smaller than in the books of the Wise Masters. Dany will at some point show up to save the day with her army of Dothraki and Dragon(s?) to wipe out the enemy army. Not quite sure how they Greyjoys will get mixed up in this yet. It might be just Euron that shows up with his thousand ships that Dany so conveniently needs. As far as the rest of the Greyjoy timeline goes I'm not too sure on that yet. No idea what Theon and Ahsa are going to do, not sure if Dany will kill/use Euron/Greyjoys just for their ships or if she will actually marry Euron.
That's a very loose and dodgy theory of mine. Still not too clear on what I think will happen or if half of that will even happen. But hey imagination if fun!