Here's a small set of proposals, definitely well thought of and not made up specifically for this comment to make a point:

Start taxing them heavily on wealth INCLUDING unrealized gains once it hits a threshold, but no wealth tax for normal people. Force companies to become either co-ops or publicly traded when certain thresholds are met - and if the founder has too much stock, the taxes on unrealized gains will force them to sell. But if it's a co-op, don't count anyone's share in it as wealth for taxation, only any profit actually paid out by the co-op. My prediction is that companies with high profit per employee (think Steam) will become worker-owned co-ops and companies with lower profit per employee will be publicly traded (think Walmart, except of course Walmart is already publicly traded)

Essentially, I want people to be able to own property, but not own so much that it negatively affects everyone else - everyone should be able to have a primary residence tax-free and I don't think it's bad for someone to own a second home either, except that shouldn't be tax-free. Hoarding property isn't OK though - that affects everyone else's housing situation. I don't like the idea of the state owning all homes - I want there to be strong rules protecting me from being evicted because the state needs a factory built right where my neighbourhood is - but there SHOULD also be state sponsored housing for those who can't afford their own homes, and they should be easily attainable, and built to a good standard.

I'm okay with people making a plentiful living from passive income off ownership in some company they built, I'm just not okay with it being so much that they make more in a year than the rest of us do in a hundred thousand.