The short answer is that's a political question along the same lines as what should the minimum wage be, or what should the rate of taxation be? There is no one right answer for all times and all situations, and it should be determined through public debate and input via voting. We as a society operate in this manner in all sorts of ways, and it's not at all inconceivable that we could do the same here.
I think the guideposts should be to put limits on how much influence a single person should have in our society, or to limit the amount of influence we unintentionally give one person just by participating in the economy. For example, if I buy a product from a company that has a large advertising spend on Facebook, I am indirectly, perhaps even unknowingly adding to the personal wealth of Facebook's owner just because of some decisions that someone else made. Now the libertarian response to that is to research each and every product you buy, but that quickly becomes an impossible task for any one person to do, much less every person. Not only is the quantity of information too large, but some information is simply inaccessible to the public at large.
So, to answer your question more directly, I can give you my personal opinion. I think capital gains should be taxed the same as regular income, inheritance taxed as regular income, unrealized gains taxed as regular income (above some threshold perhaps), and progressive taxation to asymptotically approach 100%. Right now our top tax bracket ends at 37% at $539,900 income for an individual. It's at 35% for incomes from $215,950 to $539,900. I'd call that roughly a 2% rise for a ~250k rise for back-of-the-envelope calculation purposes. So linearly extrapolating that out, maybe the top tax bracket becomes 100% after somewhere around $10mm annual income. And since you pay taxes on that $10mm, you maybe can accumulate somewhere in the neighborhood of $5mm a year. And that's generous in my opinion, because lower income tax payers pay an 8% increase (24% to 32%) to go from ~$90k to ~$170k, so it's not like its a linear function for workers.
And then how do we 'catch up' on back taxes? You kind of establish a concept of a 'maximum wage' at some point in a progressive taxation scheme that approaches 100%. I think one fair-ish way of doing it would be to cap wealth at that level, so that if the maximum income is let's say $5mm, you can keep $5mm for each working year of your life, anything beyond that being due as back taxes. So if you're 65 you could still have ~$235mm, which seems like plenty to me. And when that person dies, their children's inheritance is taxed as regular income as well and effectively capped at $5mm or so (whatever it turns out to be), so there's little point to dying with hundreds of millions of dollars in the bank.
Maybe not those exact numbers, but somewhere around there seems reasonable to me. But my opinion is just one opinion, and a big change like this should be determined through the political process with inputs from everybody.