Sidechain thats blind merge mined with a perpetual one-way peg, so you can go from bitcoin -> sidechain, but cant go back. You can do atomic swaps so you can buy the sidechain tokens if you want to use the sidechain, but bitcoin is ALWAYS more valuable. So its a way to have sidechains for doing whatever usecase you want, without a shitcoin that competes with bitcoin for monetary premium.

Could be done today with semi-trusted setup once we have new package relay with ephemeral anchors. Can be done with no trusted setup with APO or another covenant.