Historical: Sets based on real-world cultures and events. As far as I know, with the possible exception of some of the Collectible Minifigs this has never actually been a thing with LEGO, and is included mostly for completeness.
Traditional: Knights, princesses, forest bandits, and the like, with no direct connection to history but nothing supernatural, either. As befitting its name, pretty much everything from before the mid-1990s goes here, along with the first Knights Kingdom line and 2011 Kingdoms line. The late 90s Ninja, too.
Mythic: As above, but with some magic - wizards, witches, and fantastic creatures, but nothing too far from medieval legend. Dragon Masters, Fright Knights, and maybe Knights Kingdom 2, depending on how you feel about all those skeletons.
Fantastic: Magic, monsters, and what might be termed the standard fantasy races. This would be the Crown Knights era stuff with the dwarves and trolls, the Elves subtheme from LEGO Friends, and Lord of the Rings.
Gonzo: Full-blown wacky techno-fantasy - Nexo, Chima, Ninjago, Star Wars (

Personally, my sweet spot is probably somewhere in the Mythic-Fantastic range, although as I mentioned in the linked thread I wouldn't mind seeing the Fantastic-Gonzo line pushed a bit, just without being quite so . . . silly.
As an aside, regardless of your answer to the poll, what would you think about re-using the concept from the Elves of having visitors from the modern world? Let's face it, if it's not just a quick rehash like the 2013 Castle sets there's probably going to be some sort of kid-friendly storyline involved in what comes next, and it would make a nice change from the standard "five-great-warriors" trope TLC seems to like lately.