Overlays support layers. That's the general advice when this comes up.

While that may be true, the key to getting feature requests taken seriously is to explain your usecase (which you really haven't done) and explain why existing options just don't cut it, which you really haven't done.

And when I say "that's the standard advice given", I mean I've seen Martin give that advice making this same request.

And I highly doubt anything you've said so far in this thread would change that, especially with last year's increase in Mix inputs giving you the ability to have independent control and gradual transitions in and out for all 10 layers of a transparent input.

And, of course, the upcoming 27 gives you more options for controlling the positioning of Mix inputs (or anything else) on those 10 layers if the overlays you want to add need PiP rather than being set for full screen.

If that's not good enough for your usecase, which it may not be, you're going to want to provide a lot more detail (or, well, any detail) as to why.

If you need more than 4 overlays add a blank input that you show as one of the overlays, then add say 5 Mix inputs and configure these as layers on the blank input.
On the Mix inputs you can transition in any input and that will show as independently animated overlays.

Ah, OK. I'm using all 15 mixes in my show already!