Innistrad is one of the best storylines Magic the Gathering has ever had. It has a very easy to follow and well explained storyline, there aren't massive plotholes or retcons, yet it still uses characters that have previously been introduced (Way back in Lorwyn, although back then they weren't the focus of the story).
So what's next for Magic? That's what I want to determine here, and to do so by examining each of the planeswalkers:
Ajani: Last seen in Gathering Forces, Part 1. He was trying to convince Elspeth to return to Alara, which while broken, is not lost.
Jace: I forget most of what happened during Test of Metal, but that book has appeared to be written off as to have never happened. It appears he is still in control of the Consortium on Ravnica, and has been in contact with many of the walkers. Outside of this he appears to be looking to a solution to the Eldrazi
Liliana: Currently on Innistrad, and is currently seeking out the four demons who own her soul. Grislebrand is her second, and unless her role ends on Innistrad, we will likely see her again.
Chandra: Last Seen in Awakenings, Part 3. She left Zendikar shortly after fighting Jace and Sarkhan in the Eye.
Garruk: Currently chasing after Liliana on Innistrad. Whether he will break his curse so soon is unlikely, and will probably follow Liliana.
Sarkhan Vol: Sarkhan had been seen stalking Liliana for Bolas, and has more recently returned from the Eye of Ugin. He has been working for Bolas, whom he sees as some sort of idol.
Sorin: While he left Zendikar to look for a way to stop the Eldrazi, he has reappeared on his home of Innistrad to try to find Avacyn and restore order to the plane.
Nissa: Last seen on Zendikar, making a mistake in freeing the Eldrazi and then fleeing to look for help.
Gideon: Arrived on Zendikar just in time to see the Eldrazi freed. Has also fled to search for help.
Nicol Bolas: Presumably plotting to take over the Multiverse (still).
Dack Fayden: Character in the Comic Books who has landed himself on Innistrad. What he will do is left to be determined.
The Mirrodin Group
I seperate these planeswalkers because I did not read the Scars of Mirrodin Set Novel, so I am going off of forum posts. It seems however that this book might get reconned sometime in the future.
Elspeth, Karn, and Koth: Fighting Phyrexia to try to reclaim Mirrodin.
Venser: Dead
Tezzeret: Last seen allied with the Phyrexians and trying to become the Father of Machines, fighting with Glissa for he right.
So What's Next?
While Innistrad so far has succeeded in introducing Garruk and Liliana into the storyline, it has introduced yet another substory, Liliana's quest to reclaim her soul. To end Innistrad, Liliana will succeed in furthering her goal, however this does not unite the side story to the main story. Thus we introduce Sorin, who has two purposes, giving us the back-story of Innistrad (Avacyn), but he also gives us a link to the Eldrazi and the main story. Thus this leads the story to progress from Sorin in two possible directions, which I'll get to below.
There have been rumors aplenty that the next block will feature a return to Ravnica, and thus I deduct the following prediction: Ravnica remains to be a place in the multiverse where planeswalkers frequently intersect. Jace retains the Consortium there, and coming back from Zendikar he is likely to want to gather allies to fight the Eldrazi. Gideon and Nissa have also been looking for aid against the Eldrazi as well, thus come to visit Jace. Ajani too may be looking to hire aid for restoring Alara, but upon hearing the Eldrazi may be willing to let Alara be. Chandra likely will not be involved with the gathering group, despite knowing what happened there.
This leads to the last tidbit: Who comes from Innistrad? Sorin is not very welcome on Innistrad thanks to his creation, thus is also likely to join. However, while on Innistrad he is likely to align himself with either Garruk or Liliana as the book climaxes at the Helvault. In my opinion, I would think he would aide Garruk in getting his curse lifted, although Garruk will likely leave to chase Liliana after uniting with the Consortium on Ravnica. Dack Fayden will also join with the Consortium, aligning himself with Sorin.
While the driving point is toward the gathering of planeswalker allies to fight against Bolas, the plot is unwritten by Ravnica itself: What great cataclysm could occur on the plane itself that could serve to create a the backstory for the set? While an attack by the Eldrazi could work, the player base was romanced to Ravnica because of the unique guild divisions it once had, a fact which would stir scrutiny among the fanbase.
However, if the direction the story has been taken by Innistrad is any harbinger of things to come, we are in for one hell of a nice ride.
