Please tell me if I'm just being too ambitious, but I feel like there should be a way to create an NFT Contract/Collection that is not visible on the standard markets today?
I would assume it's something like Instantiate and inherit from ERC1155, overwrite some property or method, and then existing markets don't recognize this as an NFT asset?
The goal is to create a unique custom marketplace that only includes assets made specifically for it, and to prevent other markets from identifying and interacting with these assets Is this absurd?
Why would you want to do that? The point of standards like ERC20, ERC721 and ERC1155 is that a users assets are easy to interact with, that they show up in the user wallet, and that marketplaces / exchanges support them.
Its easy to make something non standard, and lose all that. But why would you?