When incorporating live elements like hosts or guests, it would be beneficial to have the option to select a shape to serve as an overlay. For instance, if there's an image displayed on the screen, and I wish to add a live feed of a guest as an overlay, I could simply choose a specific shape, such as a circle or square, for this purpose.

How would that be different ( except for circle/shapes see richardgatarski's answer) than the quick resizing/cropping etc already possible with the new (multi)layer options and overlay the result ?

And would still need to adjust size , position etc ;-)
A quick way to change a key image is to use a GT title, whenever you change/edit the title desing and reload the keying will adjust. Heck if you use multiple shapes in a title and split them up in pages to hide/show a particular shape for a quick change by selecting the page

Personally I don't have the use. Perhaps if a customer would like to have an egg-shaped sign interpreter overlayed (I hate that design).

In case you want to adjust the size and the position of the mask within vMix.

1. Use an image editor to create a black image that is larger than your production setting, say 4 times.
2. In that image, draw a white circle/rectangle/key hole/etc that is centred and less than, in the above case, 4 times the image size
3. Add the image as an input, name it "Mask"
4. Add a mix Input "Mask-flattened", and set its output to the Mask Input
5. On your camera Input, Luma key with "Mask-flattened"
6. Adjust the "Mask" Input's Position (zoom/pan/rotate)

/richard