I would love to see the ability to size/position a Luma key input just as you can adjust other input positions. This would allow you to use the merge transition to follow the luma changes to a new position instead of just fade.

This is currently not possible as it is necessary to use 2 separate luma key masks to change size and position, and merge actually does a fade between them. This is not as clean as the merge size and position change of an camera or image input. Not to mention in the current state of vMix 19.0.0.32, the settings dialog alters the luma key to the current inputs luma input for all inputs using luma masks.