What can and can't be done by having a separate settlement layer?

I have been reading about the fact that cardano has two layers, but I’m wondering what it means for the future of the computational layer. If the two layers are separate, does it mean someone could create a network similar to the settlement of cardano and use it to run the smart contracts instead of the ADA token? Or will people be able to create side chains in the future with a value unrelated to ADA? Like a sidechain pegged to bitcoin, fiat etc.