For the 'Twilight' side, you could include Bella, Edward, and Jacob. They are the main characters and would bring a lot to the story. From the 'fire and ice' world, Jon Snow would be a great addition. His sense of honor and leadership could interact in interesting ways with the Twilight characters. Also, Daenerys Targaryen with her dragons could add a whole new level of power and drama. Her dragons might be seen as a threat by the vampires or werewolves, or they could form an alliance.
Well, in 'fire and ice twilight fanfiction', characters like Carlisle from Twilight could be really interesting. He's an old and wise vampire, and his knowledge could be valuable in the 'fire and ice' world. From the other side, Arya Stark would be a great character to include. Her skills as an assassin and her ability to adapt could lead to some really cool interactions with the Twilight characters. And of course, you can't forget about Jacob Black. His werewolf nature could have some unique reactions to the magic and creatures in the 'fire and ice' world.