Well, here is a quick beginning brain dump with one new idea from today.
1. Only single purpose corporations allowed. (to be traded publicly???) (And the purpose must be narrowly defined.)
2. Only real live humans can own common or voting stock in corporations. Other non-human legal entities can only own stock that cannot vote.
3. No corporation will be allowed to grow to be big enough that the country cannot afford to let it fail. (This is the new idea from today.)
Link from Mike's comment below:
Competition in the Banking Industry