Big-hearted billionaires could be one of the biggest limiting factors to the creation of a trillionaire. Buffett and Gates have been the most vocal with their Giving Pledge campaign to convince the world's richest to leave a majority of their fortune to philanthropic preferences.
"This Buffett/Gates approach has broad appeal," Kraus said. "Really wealthy people now are taking greater steps to distribute wealth in their own lifetime. ... To extent the extent that there is a limiting factor on someone becoming a trillionaire, this might be it," Kraus said.
Another way to think about Bill Gates' focus on nonprofit work is, simply put, boredom. "If you take an ambitious, talented person with a zero net worth, accumulating wealth would be reasonably high on their list of priorities," Lord said. "But as net worth goes up, most folks lose interest. I doubt Bill Gates thinks much about accumulating more wealth these days, and he certainly doesn't dedicate his time and energy in that direction. ... The same could happen to Elon Musk."
But philanthropy may not serve as a limiting factor for all, and in the end, this may just ensure that the greedier billionaires will take the lead from Buffett and Gates. "Some people, like Sam Walton and the Koch brothers, they never seem to lose focus," Lord said. So the boredom factor ultimately will not preclude a trillionaire—and not necessarily lead to a Giving Pledge commitment.
Read MoreWill Wall Street still be around in 25 years?
If the U.S. is going to be able to boast the 21st century's first trillionaire, tax policy will have to continue on its current path.
When Ronald Reagan took office as president of the United States, the top personal income tax rate in the U.S. was 70 percent (91 percent back when Eisenhower was in office). Tax policy in the U.S. in recent decades has figured prominently in wealth accumulation.
Kraus sees few signs of this trend reversing. "From a social standpoint, the rich are getting richer, yet the notion of social mobility is so central to the fabric of America, and people buy into that even if it's against their own best interest." Income inequality and redistribution of wealth have not been in the top set of considerations for voters. "I don't see a pressing desire by the electorate or D.C. to change," Kraus added.
Oxfam conducted an assessment of how much presidents talk about income inequality—or proxies for that term—in State of the Union addresses over a hundred years, and found a massive amount of variation, O'Brien said. "There were eras where if you didn't talk about it you had no chance of succeeding. In 1912, we had four progressives and a socialist running for president."
Read More For super-rich, investing to see dramatic changes
Since LBJ's war on poverty there has been a significant falling off in that conversation, and that trend exacerbated under Reagan.
"The money will have to come from somewhere to deal with debt and the deficit, so the 'tax the rich' philosophy will make some gains" but it won't reverse the trend and will only limit even greater concentration, Kraus said.