Set a cap of $1 trillion or $500 billion that any single firm can have under management. This probably only breaks up Vanguard and BlackRock. Not sure how this would cause havoc. Seems like we need to relearn the reason anti-trust laws we passed in the US and start using them again.
I do think Vanguard has been a great boon for American investors and would not like them punished for their success. Not sure how to square that fact with the need to break them up.