If you want obviously thematic powers, Cold Snap, Ray of Frost and Entropic Winds all do Frost damage. Since they also all inflict Frigid, they'd be good abilities to pair with Ryn for extra damage via Twist the Knife (and if you want to take advantage of it yourself, make a 'Magic Thief'' build) which sounds like you'd also like it since it means Ryn in the party. Oh, and Frost Arrow is another obviously boer'alvar power if you want to have an archer-type character and are fine with beating up Hethia and stealing her bow.
Otherwise there are some builds you could do to copy the feel of Alissa who seems like she'd be a good representative of a boreal elf warrior (Driving Thrust, Steady Strike and Thunder Strike), possibly with a spear for the thematic factor. Or you could make builds around Elthara's scythe or Jael'yn's greatsword. I for one am looking forward to whatever we're able to do with the latter once the quest to restore it to its proper state gets implemented.