Probably too early even for this discussion, but why not: https://github.com/cconvey/cpu-n1

It's a simple (WIP) ISA and corresponding simulator. I want to get experience adding a new backend target for LLVM, and this is will be the target.

The idea is to be similar to other LLVM targets, but different enough that I can't just copy-and-paste existing LLVM code.