I don't think it's a weapon, or at least a top tier weapon. They way Luke Fiddzz worded his response about it was (paraphrasing, can't find the post): "It's something, at least I think, that is very cool"
The "At least I think" implies there could be some who don't like it. There is no one reasonable who wouldn't be happy with Hakkon weapon/top tier weapon. I suppose it could be the ham mace, the spoon, or some other joke item like the cheese wheel. But I'm going with new armor skin(s) for the characters.
EDIT: Found it.
It was Fiddzz, not Luke. Only bits we have to go on:
"We said there would not be much special stuff in this book, i think there was a dragon call for star 3, and that future books would have more rewarding... rewards."
"The next books 5th star reward is (i think) really cool."
"In the next book X1/PC/PS4 will be getting something very different in the final chest that is pretty cool. X360 and PS3 will get a bonus superb requsitions chest at star 5."
Soooo...it's something that is pretty OR really cool. Could be a coupon for a free blizzard at Dairy Queen for all we know.