I would suggest to provide (additional) parameters for the Crop: circle filter which would greatly improve its usefulness, especially in combination with keyframes:
Center
Radius
Blur edge
Center of course would need some X and Y value, which would be nice for the radius, too, factually enabling ellipses. However, for maximum use of ellipses this would also require a rotation/tilt then. When allowing off screen centers and changes in edge blur over time with keyframes, this may enable many cool effects and even transitions in combination with transparency. BTW, edge blur might be a nice addition to the Crop: rectangle filter as well.
Guess you guys are right that my ideas can be realised already. Though I am not sure about the use case for the crop circle/rectangle filters any more. Maybe this confusion was the origin of my idea in the first case. Just ignore the proposal.
The crop circle and rectangle filters were made to replace some even older filters based on a legacy technology that had to be removed (WebVfx), well before Glaxnimate. They are still somewhat useful for simple tasks like to make a colored border or circular video. To do that with Mask: Simple Shape requires some track layering or something else not obvious. Of course, you can make these crops also do layer/filter masking, but as @Namna pointed out there is Mask: Simple Shape for that. Glaxnimate integration takes both things to the next level: Open Other > Animation as a border overlay, Mask: Draw for more complex mask shapes. I prefer to keep the simple things simple instead of growing them towards something redundant.