I think it'd be nice for Castiel to officially meet Hiccup's mom and Cloud Jumper(could be while the teens are there or after they leave). Might be even funny for her to do that thing parents normally do where they embarrass their kids in front of people they like haha(though thats your choice if you want her to or not, jut thought it might add a littl bit of comedy XD).
Now, as far as an idea. I think we should do something having to do with Castiel's old village. Any suggestions?