According to Elder Scrolls lore children of mixed parentage generally resemble their mother, so there is nothing wrong with using the basic racial templates: if your character's mother was a bosmer and their father was a breton then create the character as a bosmer and let their backstory influence how you play (maybe they're magically inclined).
How this ingame book was called? I remember reading it in TES3, but I forgot the name?
Anyway while creating race there should be possibility to choose mother/father
Your character inherits 2/3 of mother race traits and 1/3 of father traits.
Oh and one more thing, for anyone who thinks about mixing khajiit and argonian with mer/men races (or one with another), I simply must disappoint you.
Beast races can't breed with other races under normal circumstances.
Maybe (just maybe), it could be achieved with assistance of some sophisticated magic (that could have effect similar to modern gene splicing)