A fantasy theme like what Jetrax99 suggested would be cool. I'm all for mythical creatures, and that would open up a nice opportunity for human skin tones and hands and feet, with fairies. That could also open up the line to less friendly creatures, like centaurs, dragons and ogres.
Does anyone think that is possible that we could get Makuta and the MoUP in a fall set this year? I still have a hard time believing that we won't get either of them, especially after being in JtO. Maybe that's just wishful thinking, though.
Expanding it to all animals, not just arthropods and dinosaurs would force them to start making more organic-looking sets. That would be nice to see. Scale, hair and feather add-ons would be a given, anms would more natural colors, browns, earthy colors, and other colors we don't have much of yet.
So I went to a TRU in AZ today, and they had all 5 sets. I don't have any spending money at the moment, so I couldn't get them, but I wonder how long they will stick around. There was only one Storm Beast, and multiples of everyone else. I have not personally seen any evidence of Bionicle's decline either, as they were also well stocked with the Summer 2015 and Winter 2016 sets. Very interesting.