You make "almost illegal" financial activities really illegal in the law, prosecute people, and I suspect that 90% of the problems will disappear automagically. Number of people who can become billionaires without committing borderline illegal and completely illegal activities are vanishingly small I suspect, probably only founders of megacorporations and those are often doing illegal stuff too to stay ahead. USA is an outlier probably as far as honest successful corporations go, but in other countries if you are a top100 Fortune person you are likely a criminal too. In my country for sure. Energy - illegal oligopoly, metallurgy - illegal oligopoly, animal farming - illegal oligopoly, etc. The number of illegal schemes happening in the big corps in my country is staggering.
Not to defend corruption, and I'll agree absolutely that there's a lot if room for improvement, but far too many people break far too many laws for full enforcement of any of them; and even outside egregious violations, law itself is too esoteric for most people to understand either their obligations or their rights.
With regard to the first part, the normal examples I give of this are road traffic offences (which if fully enforced would mean the only people allowed to drive would be those who didn't); and drug laws — ignore weed for now, nobody defends heroin, full enforcement of UK heroin laws would treble the UK prison population all by itself, while enforcing the laws on cocaine would more than double the US prison population and increase the British one by a factor of 20.
With regard to the second, look at how confused people get about copyright, trademarks, how they work, what the penalties are for violating them, what counts as "fair use", and so on.
We should focus on the large scale offenses by big entities of course. E.g. if you have 50% market share of chicken production in the country (and another 50% is also a single corporation) then I'm sure that both of the corporations are breaking a lot of laws and utilising a lot of barely legal schemes to launder profits. Same for all sectors of economy. Examples closer to us - ISPs, Cell operators, providers of cloud computing, search, communications etc. All of that must be carefully examined, and then partitioned where monopolies or conflicts of interests arise.
