Fault-Tolerant mainframe type systems.

https://en.m.wikipedia.org/wiki/NonStop_(server_computers)

They are amazing machines designed for fault tolerance (99.999% reliability). The Wikipedia article below has design details for how many generations were made. HP bought them.

https://en.m.wikipedia.org/wiki/Tandem_Computers

I think it would be useful in open-source, fault tolerance to copy one of their designs with SiFive's RISC-V cores. They could use a 20 year old approach to dodge patent issues. Despite its age, the design would probably be competitive, maybe better, than FOSS clusters on modern hardware in fault tolerance.

One might also combine the architecture with one of the strong-consistency DR'S, like FoundationDB or CochroachDB, with modifications to take advantage of its custom hardware. At the local site, the result would be easy scaling of a system whose nodes appeared to never fail. The administrator still has to do regular maintenance, though, as the system reports component failures which it works around.

Not only does NonStop not support Rust, but apparently they failed to port gcc to it, even. So compiling Rust straight to C itself is pretty much the only option there.