The obvious Frozen, Avatar (TV series), Rise of the Guardians crossover that 20,000 people have probably already suggested

I find myself wishing I had enough available funds to buy cold medicine so first, here's a link to a past post in which the post zombie apocalypse narrator says that what he misses most from civilization is cold medicine.  Second, just a quick post here.

Before I ever saw Frozen, Lonespark told me that, being an Avatar fan, she saw Elsa as an Ice Bender.  Water Benders have an affinity for ice and while I don't think there are any examples where someone can Ice Bend without being able to control liquid water it's certainly not outside the realm of possibility.  So, then, what Elsa needed was a mentor in the art of Ice Bending to help her control her powers.  (Stark contrast to the hide from the world and try to never use her powers plan her parents set her on.)

I suggested Jack Frost, from Rise of the Guardians.  I was told that he and Elsa would make a good fit and now that I have seen the movie, and even written about a theme it shares with Wreak it Ralph, I definitely agree.

What Elsa needed growing up was to be taken under the wing of Jack Frost and tutored in the role of controlling ice.  More fun, less depression/isolation/loneliness, and moreover more control.  By teaching her to use her power she wouldn't end up having it slip out when she got frustrated/aggravated/scared/angry.

"Do you want to build a snow man?" would come the question.

"Yes!" would come the reply.

Jack, the immortal flying Ice Bender could help Elsa lead Arendelle in the treacherous world of Fire, Air, Water, and Earth benders.