- Avoid requiring the user to figure out how to get into BIOS/EFI and change boot order. Windows has APIs for manipulating EFI things, may be worth looking into that.
- Replace GRUB with something more modern like rEFInd or Clover with a nice looking theme.
For the latter point, while GRUB is technically functional, it looks scary and arcane to new users and has little resiliency to things like Windows updates mucking with boot entries. It makes for a bad first impression (“why is my computer showing hacker screens suddenly”) and when it breaks your average user doesn’t have a prayer of fixing it. Something that looks more modern and self-heals would be a big improvement.