I used to be a 'psionics isn't magic' guy in 2e but I got over that early in 3e... but
but is your limit "Something I can show batman doing in a comic/movie/cartoon" or "Something I think a human can do even if world records indicate otherwise"? If it is the former, I can get behind it. if it is the latter I can not (and of course replace batman with any non magical non supernatural/enhanced hero in fiction.)
every hero has an origin...
we give wizards more power then gandalf ever showed (let alone ratagast) and he was a god/angel. we give wizards more power then jack vance did. we give wizards more power then merlin who was an immortal ageing backwards half demon...
if you need a 'cool origin' to get supernatural powers, then no one should be able to play wizards.
every hero has an origin.
my buddy plays the same family line all the time... he is a human fighter who is descendent of a silver dragon (becuse way back before I was playing in 91 he played a game where his character married a silver dragon and had a family and he like playing the grand children, great grand children ect) If he was playing a spell caster he can play a sorcerer (silver dragon blood line) and get cool powers. If he wants to play a martial character there is not a class for "My origin for my warrior is that I was born fantastical"
I'm an elf, and desendent of lolth... I'm an aasimar, I'm a teifling, I'm a dragonborn, I'm a ______.... my great grand dad on my mom's side was a demi god and my great grand ma on my dad's side was directly blood of the legendary king... both powerful blood lines are in me....
I was just a farmer... nothing special, until a rock fell from the sky killing half my family, it glowed green and red and the half that didn't die in impact all got sick... except me, I was always hardy and tough, I took the metal and forged my first chain shirt and sword and went out into the world... I am now destine for greatness.
I am the 7th son of the 7th daughter of the merging of a teifling woman and an aasimar man... I have a strange power...
I fought my way up the mountain and learned my sword techniques from the old man who even though he was human but over 100 years old and still could kick my 18 year old butt...
but it should be up to the table/people what they do and don't need... do you ask for justification for my farmboy wizard to just gain 2 6th level spells known at 11th level?