If you make it an "L" lens with IS, it's going to cost well over $1000, which kind of defeats the purpose of its existence.
IS adds cost and size to a lens, is it really needed with IBIS bodies becoming the norm? I'd rather a top end focus motor... After using the RF 85 f/2 IS... give me usm (or linear?) over IS any day.
Look at the EF 85 1.2L II vs the EF 85 f/1.4L IS.... they really weren't all that different in price ($1600 at launch for the 1.4). The 1.4 was just a needed modern take on a fast 85L. The RF 50 f/1.2 is already a modern design with fast autofocus.
I'd like to see such a lens with some level of weather sealing, but if you want it sub $1000, it can't be an "L".