Reading through this thread, I was thinking of a comtime signed integer (basically a UInt whose sign is given by a parameter). I think this could solve some of the issues mentioned here. Nonetheless, I would be surprised if other people had not been thinking about this already, so I would appreciate comments as to why this is not a good option.
To keep the current thread focused, I would propose to discuss this here, where I also provide some more details and a reference implementation: Pre-Proposal: a comptime signed int for indexing