*Warning: Worthless speculation ahead!*It's absurd how little we know about where those subplots could go. I guess Mira's is going to be scheming to screw over the Whitehills, but who the fuck knows where the North Grove train is going. We don't even know why it's important and what it can do, if it can do anything at all!
I feel like Mira's story in Season 2 is just going to be a mimicry of Sansa's:
an episode of trying and failing to worm her way out of Morgryn's marriage pact, an episode of trying to make new allies amongst the people around Morgryn, an episode in which they are wed, an episode in which the marriage completely goes to hell (Whitehill-related?), and an episode in which she flees Morgryn and tries to make her way back to the fam, with nothing to show for her endeavors.
As for Tuttle Heart-Eater:
My wild guess is that his arc will roughly be: help the bastards fight undead, help them fight more undead, encounter Crows and try to justify his past actions, meet with Jon and try to negotiate an understanding, get caught in the middle of wildling bullshit and earn nothing... just in time for Season 3!