I was thinking something similar this morning, but a full stop improvement so the Canon RF 100-500mm f/4.5-7.1 L IS USM Lens ($2799 currently) becomes the:
Canon RF 100-500 mm f3.2-5 L IS USM (? $4499-4999)
which would compete with the newly released Sony FE 100-400 F4.5 GM OSS ($4298)
While the Sony is a constant aperture versus a variable for the Canon, with auto ISO, to me a variable aperture isn't a big deal, if the Canon is a full stop faster at the short end and probably about the same at 400
The entrance pupil is 100mm, less than the 107mm on the 100-300 f2.8 and the proposed 300-600 f5.6, so smaller and lighter than the 100-300 f2.8
If it is built so it can use the extenders:
1.4x converter gives you a RF 140-700 mm f4.5-7.1 L IS USM
2x converter gives you a RF 200-1000 mm f6.3-9 L IS USM
which gives you an extra 200mm at the far end over the non-L
Canon RF 200-800 f6.3-9 IS USM