Thinking about ultra-fast lenses (ƒ<1), it would seem useful to have a solid+liquid medium between lens and sensor. This wouldn't have been very practical (presumably) at the time of film, but with a digital sensor I don't think there's much issue with coating it with transparent glass. Removable lenses would be somewhat cumbersome, but not hopelessly so with the right index-matching fluid (as common in high-NA microscopy). Of course lens designs would have to be altered but I'm curious as to why this is not a thing (that I know of, beside a few esoteric mentions here and there)?