Hanson are now preparing for the Wintry Mix Tour, combining Christmas songs, hit singles and Against The World songs in one show. Great idea! But how did the Wintry Mix concept start?
This blog post from October 2016 reveals that the 2017 Christmas album was going to be called... Wintry Mix. I guess Hanson ust took that concept, thought to hold it off (maybe the idea of the forthcoming tour was already in it's formative stages then (remember initially they were talking about the turnaround between album 6 and album 7 being one of the shortest ever -- which turned into the longest by a long shot).
Or maaybe (possibly) it was shelved because the idea for String Theory presented itself.