Transatomic claimed to have the fuel utilization of fast reactors without being a fast reactor. That turned out not to be the case.

However, there are a bunch of other startups working on molten salt reactors, including Bill Gates' Terrapower, Terrestrial Energy, Thorcon, Moltex, Elysium, and Flibe. The farthest along is probably Terrestrial Energy, which has gotten through the hardest part of Canada's licensing process.

I would really like to see the particle beam fission concept explored also due to its versitility and control:

https://en.wikipedia.org/wiki/Accelerator-driven_subcritical...

It's a niche technology, and doesn't address the main problem of fission (that the reactors are too expensive). At least it serves as a counterargument to those who want to make fusion-fission hybrid reactors.