What about taking inheritance and gifts as ordinary income to the recipient? Ideally this would be coupled with a reform on capital gains tax, but even as is, should target high taxes only on large amounts or amounts given to people that are already rich.
There would certainly need to be some tweaks, but it should diminish the inherent injustice of inheritance.
You can treat all gifts a year or less before the date of death as inheritance. Many wealthy parents will be hesitant to give large amounts of money to children while in good health, as they're scared children won't care about them any more once they get their money.
The bottom line is, if someone wants to leave a significant inheritance to their (grand)children, there's little that can be done about it with tax policies only. By the time you have plugged all the holes to prevent inheritance, you've also prevented most of the things we consider necessary for the function of a capitalist free economy.
For 100% (or otherwise significant) inheritance tax, the issue is how to implement it. If you know it's happening, you'll just gift what you own to the preferred recipients before you die. If gifts are also 100% taxed, you sell your property to them (and then spring up a company and hire them to do "work", so they get the money back). If sale of property and work are 100% taxed, well, it's a very different kind of economy.
100% inheritance tax only hurts when someone dies unexpectedly. Others will be prepared with workarounds.
Where I live, both inheritance and gifts are taxed with very similar rates. Also, selling things under their market value is considered gifting (which sometimes gets interesting, for obvious reasons).