I wonder if there's any possibility that an RDMA expansion device could exist in the future - i.e. a box full of RAM on the other end of a thunderbolt cable. Although I guess such a device would cost almost as much as a mac mini in any case...

RDMA is not really intended for this. RDMA is really just a bunch of functionality of a PCIe device, and even PCIe isn’t really quite right to use like RAM because its cache semantics aren’t intended for this use case.

But the industry knows this, and there’s a technology that is electrically compatible with PCIe that is intended for use as RAM among other things: CXL. I wonder if a anyone will ever build CXL over USB-C.

You still need an interface which does at least two things: handles incoming read/write requests using some kind of network protocol, and operates as a memory controller for the RAM.

Texas Memory Systems was in the business of making large 'RAM Drives'. They had a product line known as "RamSan" which made many gigabytes/terabytes of DDR available via a block storage interface over infiniband and fibre channel. The control layer was implemented via FPGA.

I recall a press release from 2004 which publicized the US govt purchase of a 2.5TB RamSan. They later expanded into SSDs and were acquired by IBM in 2012.

https://en.wikipedia.org/wiki/Texas_Memory_Systems

https://www.lhcomp.com/vendors/tms/TMS-RamSan300-DataSheet.p...

https://gizmodo.com/u-s-government-purchases-worlds-largest-...

https://www.lhcomp.com/vendors/tms/TMS-RamSan20-DataSheet.pd...

https://www.ibm.com/support/pages/ibm-plans-acquire-texas-me...

Couldn't you "just" use a honking fast SSD and set it as a swap drive?