There is absolutely no reason at all to believe that Sarkahn is the other Walker....
Nicol Bolas is interested by the Veil, and Sarkhan is following Liliana, so it's possible he is in Innistrad. (I'm not saying it's gonna happen, I hate Sarkhan, but it's possible)
To be honest, there is only one character mentioned who is really mysterious and potentially important in this story : Jenrik, the astronomer from Nephalia.
I don't know why, but when I read this : "Wards keep away werewolves, and the Stromkirk actually fear his knowledge, for anyone with such a vast understanding of the moon is holding great power indeed. Some say he is predicting the future of Innistrad, or that he is a spirit trying to get home. Others say he is an angel attempting to restore Avacyn, or that he is a demon plotting to destroy the world.", I can't help but think is gonna do something in AVR.
He's also quoted in the flavour text of Tower Geist : "Jenrik's tower is served by those who once sought to enter it uninvited".
I can't tell for sure he will be in AVR, but to be honest, that wouldn't surprise me. And I would love it. A lot.
Giant aquatic beasties of the serpent/leviathan/kraken variety have already been confirmed to exist in Innistrad. It was mentioned in passing in one of the Savor the Flavor articles concerning the setting.
"The River Kirch
This wide, fast-flowing river originates in the mountains of Stensia. It empties into the Lake of Herons, with murky water, high ridges bordering the bank, and depths of hundreds of feet. Great sea serpents and other creatures are said to hide in the depths of the Lake of Herons, which stretches almost 20 miles before flowing over the 2,000-foot waterfall known as Kirch Falls."
About squirrels : in the Marvel Universe, Squirrel-Girl has a unique power, full control over... squirrels. It may sounds a little weak, funny, childish, and not impressive at all, but she managed to defeat several big vilains, some of them twice.
Even if I can't give you any source (because it's a really old post on Maro's tumblr, and I don't have the time to look for it right now), I'm pretty sure it has been said that Ludevic would never appear on a card (I think Maro said it was one of the characters of Innistrad he liked the most).
But I'm really intriged by "Jenrik", a mysterious astronomer of Nephalia, feared by many monstruous inhabitants of the plane...
I would like to see something like a summoning circle : a land, who gives you black mana, and if you spend some life points, it transforms into a demon (you have to sacrifice some blood, or maybe one or two creatures, to complete the summoning).
Nicol Bolas is interested by the Veil, and Sarkhan is following Liliana, so it's possible he is in Innistrad. (I'm not saying it's gonna happen, I hate Sarkhan, but it's possible)
I don't know why, but when I read this : "Wards keep away werewolves, and the Stromkirk actually fear his knowledge, for anyone with such a vast understanding of the moon is holding great power indeed. Some say he is predicting the future of Innistrad, or that he is a spirit trying to get home. Others say he is an angel attempting to restore Avacyn, or that he is a demon plotting to destroy the world.", I can't help but think is gonna do something in AVR.
He's also quoted in the flavour text of Tower Geist : "Jenrik's tower is served by those who once sought to enter it uninvited".
I can't tell for sure he will be in AVR, but to be honest, that wouldn't surprise me. And I would love it. A lot.
Edit : too slow
"The River Kirch
This wide, fast-flowing river originates in the mountains of Stensia. It empties into the Lake of Herons, with murky water, high ridges bordering the bank, and depths of hundreds of feet. Great sea serpents and other creatures are said to hide in the depths of the Lake of Herons, which stretches almost 20 miles before flowing over the 2,000-foot waterfall known as Kirch Falls."
So... Never underestimate a squirrel. Never ever.
...
Ok, he's very dead.
Ugin, the Dragon-Spirit, who helped Sorin and the mysterious lithomancer keeping the Eldrazi on Zendikar !
But I'm really intriged by "Jenrik", a mysterious astronomer of Nephalia, feared by many monstruous inhabitants of the plane...
( http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/stf/165 , they talk about him )
You can pre-order it in this french website : http://www.soylentcomics.com/107248-magic-the-gathering-issue-1.html
But I don't know if they'll give you the promotional card.