Polaris and Quicksilver
Right now, Mike Carey's hinting that there'll be Polaris and Magneto interaction this year and I do want to read that but I've seen those two together already. I really want to see her interacting with the twins, especially after House of M. That sort of story should completely change their dynamic.
Of course, the big character dynamic with the twins was set up in a lackluster way by Chuck Austen in Uncanny X-men. Prior to that, I don't remember her really dealing with Wanda (I'm trying to put together the issues of Exiles where AU Wanda and Lorna were together) but she's had a lot of contact with Pietro. They were on X-Factor together, and later they both worked in Genosha for Magneto. Sadly, I think he left before she found out they were related because Austen has her dropping the news on them after her initial post-Genosha breakdown, during the build-up to that dreadful Wedding storyline that I hope Wanda wipes from reality in Avengers: Children's Crusade #5. It bugs me a bit that we didn't see a significant team-up between Pietro and Lorna during that.
Don't get me wrong, I'd love to see the sisters together, but Pietro and Lorna were on a superhero team together before they found out they were related. They worked closely for a while, and I want to see how this discovery affected this. Not only that, Quicksilver is an extremely family-oriented character, and Polaris has a running plot about trying to discover her true self. These characters should want to have a relationship and talk about this revelation, how it's affected them, and just generally about the family.
Lorna is the innocent member of the family in the House of M family fight fall-out, but she was also pulled into the family substantially for the first time by the twins during this. Having been present at the climax in HoM#7 she should remember Wandaworld, her father's fit, and have some words for both the twins and Magneto. That's a mess of emotions on her end. There's huge dramatic potential here and honestly, the best time to do it is during this period between Secret Invasion and Children's Crusade while Pietro is sane but miserably alone and disconnected from his family. (When Wanda gets back, Pietro won't be nearly so starved for sisterly approval.)
Sadly, Brevoort's formspring answers suggest there is no interest in developing this in the Avengers books, and the X-books for years have focused on Magneto and Polaris over her relationship with either twin. I have my fingers crossed Peter David will do something in X-Factor, though.