I have no idea about the state of modern mathematics but I wouldn't have guessed that the idea of a lone mathematician has passed. What are the big projects in mathematics?
I'm not sure if it's necessarily big projects but the proving difficult theorems today has often involved the construction of huge "machinery", whole branch of math, that then get applied to simple-to-state-but-difficult-to-prove theorems, the example being Wiles using modular form theories to prove Fermet's last theorem.
And this situation comes because all (or the great majority) of the easy theorems have been proved for most established branches of math.
This also means great discoveries are coming at a later age for mathematicians, as simply getting up to speed in complex fields takes years.
All of this implies it would be hard to have another Von Neumann today.
It would take people like us (anyone who doesn't already know the big projects in math are) years of study to resolve that list in to anything more than names. However if you're looking for a modern math celebrity I'd volunteer Terrence Tao. Although only a mathematician could understand what he's working on it's clear from how he's talked about that in eighty years people will be saying, "I wonder if there will be any people like Tao in my lifetime."
And this situation comes because all (or the great majority) of the easy theorems have been proved for most established branches of math.
This also means great discoveries are coming at a later age for mathematicians, as simply getting up to speed in complex fields takes years.
All of this implies it would be hard to have another Von Neumann today.