As a non-book reader, I don't know who any of the new characters are or anything about the characters who are seemingly left out, but I'm on the side of introducing as few new characters as possible to the show from here on out. The show runners have said they only want to do 7 seasons, 8 max. So, assuming the final season is dedicated to some huge war between the humans and the ice zombies (that's where this has to be going, right?), that leaves only two seasons to get all of the pieces in place for that. Season 4 ended with things in such disarray that I would hope they start working on pulling things back together in the next season to set up the final confrontation, not adding more story lines that need to be wrapped up before the end.
Again, as I've mentioned a couple times in this thread, I'm scarred by the experience of reading Jordan's Wheel of Time series and don't want the same thing to happen the GoT show (Martin can do what he wants in the books...I'm not getting sucked into reading those until they're all complete).