So we know Kristoff hangs with the grown up ice men, and obviously someone have him a little sleigh and reindeer but there's no suggestion directly who or if they're alive but also if they aren't.
Then he follows the ice trail after the King and Queen, is spying when Mama Troll saus "oh cuties, I'll keep you". We don't get to hear what he has to say, but we know he subsequently grew up with them.
Aibu to think that Frozen Two ought to be a look into Kristoffs childhood, a reunion with his parent's who assumed he had been eaten by wolves, the trial of Mama Troll for kidnap charges which will reveal Trolls to the rest of the world and a cliff hanger as to whether she makes it to the wedding of Kristoff and Anna?