I suggested to two scope company reps at a gun show some years ago that they put a spirit level in their scopes. Place it so it appears at the bottom of the field of view with the bubble centered on the vertical wire when the scope was level. Both reps though it was a good idea. US Optics has one in some of their scopes.
Konus has scopes in the $300 range with that feature. I'm surprised more don't.
I wrote in a similar suggestion to Bushnell a couple of years ago- but suggested it be electronic.
Three little dots... green one in the center, red on either side- indicating cant. Green one lights up when scope is perfectly level.