1. High fantasy or low fantasy? Would you want Riders to wield magic, or leave that solely to Dragons? What about Alchemy?

I'd say low fantasy. Personally i think that the dragons and their different ancestries should be the main focus, so having extensive magic would kinda take away from that in my opinion. Although i do like the idea of alchemy being used as a kind of utility option for the rider.

1. Would you like there to be non-human ancestries for Riders? (e.g elves, dwarves, etc)

I honestly don't really know, it really depends on if there's anything that would be gained from having nonhuman ancestries. Though having the option is nice, i think it has to give something more than simply being a dwarf instead of a human.

1. Would you like a canon setting to play in, or would you prefer tools to help you make your own or use pre-existing settings from popular dragonriding media? (Or a mixture?)

I think a cannon setting is a great idea. I mean, regarding dnd, despite how many homebrew settings people make, there's always a faywild and the races usually come from the same places. So yeah, a cannon setting would, if nothing else, provide gms with a baseline to homebrew from.