Aaron posted the following on a recent BGG thread that was speculating on the next Arena expansion.
"I'm almost 100% positive the next expansion will have Frost damage spells in it. It's something we originally meant to have back in Druid vs. Necromancer, but it ended up not fitting with the Mage we were making. However, unless something pretty drastic happens I'd expect Freeze/Frost Damage to be in the next set."