Many moons after I made the proposal, @hiiamboris has proposed the same idea for Red (in a way that is a lot harder to read):
https://github.com/red/red/wiki/[PROP]-Series-evolution
He agrees generally on the need for more invariants, but specifically the idea of an unconstrained index:
"An idea is to allow series' offset ( index? ) take any integer value."
I've already tried to tackle the problem with the FIND matrix a bit: