I just got this from a completely new source. So there is a VERY big chance it might be wrong and complete BS. But I am posting it nevertheless because I am curious to learn from you if actually an E-mount to E-mount Speed booster would be even technically possible to make! Here is what that message contained:

APS-C will be pushed BIG in the near term.  Better high-end APS-C glass, and higher resolution sensors.  FX-30 is recent proof of this.  Will Sony release a A6800 or A8000 ??  I don’t know, but the next rumor is ‘quasi’ related to the APS-C rumor.

Second:
Sony will release a 500/4 lens, and a speed booster in 2023.
The new FE lens is coming as the global chip shortage is almost gone. (Chip bloat is coming)
It won’t quite be 500mm, but actually 485, but sold as a 500mm.  F-stop will be 4.12, and not a 4.0, but this new GM lens will be sold as a 500/4.  I have no idea on pricing, but probably between €7k and €9k.

Speed Booster will concentrate Full-Frame FE down to APS-C E-size.  Speed booster is intended to boost the APS-C camera market.  Hopefully the E-Mount speed booster will work for ALL FE lenses, not just GM lenses.

Like I said, E-mount to E-mount Speedbooster really sounds a lot like BS…but if it would be technically possible I actually think this would be a great product to make!