the concept of this next gen universe is to take notable merchandise-exclusive ponies with interesting color schemes & designs and try to fit them all in as children of various pairings who might make sense as their parents

name: Princess Dawn Flash (goes by "Rainbow Flash")
parents: Twilight Sparkle, Sunset Shimmer, & Starlight Glimmer (Twilight/Sunset genetically)
personality traits: cheerful, determined, imaginative, careless, stubborn

background:
From a young age, Dawn Flash idolized her super cool 'aunt' Rainbow Dash, to the point of eventually demanding to be called Rainbow Flash instead of Dawn (much to Rainbow Dash's amusement.) As a filly, Flash wanted to fly alongside her idol whenever possible — and was not satisfied with just asking one of her parents to cast the wing spell on her whenever they had time. Though Flash was never very naturally magically talented, her desire to cast spells beyond her capabilities led to an interest in potion mixing, and eventually a career crafting and selling potions that function as more basic replacements for common or semi-common spells that normally might not be possible for the buyer to cast, whether because they're unicorn, pegasi, or earth pony specific, or just because of skill level. Though Rainbow Flash has outgrown her childhood idolization of her 'aunt,' the nickname has stuck with her, although she does tend to go by just "Flash" when Rainbow Dash is around to avoid confusion.