Just had a thought while discussing primes vs zooms, utility vs image quality and cropping from a prime vs using the full frame with a zoom.
The usual teleconverters available are 1.4x (1 stop) and 2x (2 stops). Nikon also produces a 1.7x version (1.5 stops). Beyond 1.4x, these tend to come with a significant tradeoff in image quality, despite magnifying the central, sharpest portion of the image circle, and are most useful when shooting action with long primes.
But what about smaller magnifications? 1.15x (0.33 stops) and 1.3x (0.67 stops), or a 1.22x (0.5 stops) TC, would carry negligible IQ penalties when used with sharp lenses and, together with a 1.4x TC, would greatly increase the versatility of primes, particularly if you don't need the rapid FL changes that a zoom provides, but want to utilise as much of the sensor area as possible rather than cropping, to maximise IQ (which is the point of using a prime in the first place). Pair 1.15, 1.3 and 1.4x TCs with a sharp prime setup and you'd end up with a kit that's sharper than a zoom setup, no heavier and with just as much focal length versatility. I could certainly envisage a setup with multiple Otus lenses and TCs being an ultimate non-action kit.
1.1-1.4x TCs already exist for projector lenses - why not photographic lenses too?