I think generalizing conformance macros to extension macros is a good idea, but it should also be possible to attach attributes and modifiers to the extension. Attributes in particular are crucial for declaring conformances with partial availability, a feature ConformanceMacro cannot currently support.
Better yet, the tuple should be replaced by a nominal type that would allow these kinds of changes in the future as new features are added to the language. The other expansion methods all return a complete syntax node per item, which means they effortlessly support future language features. But conformance/extension macros instead return a tuple of constituents of a syntax node; expanding that tuple will cause a source compatibility break. There are good reasons not to just return an ExtensionDeclSyntax, but that means we need to think about other ways to provide the flexibility it needs.
(Alternatively, just pass a TypeSyntax node to the expansion method and require it to return only ExtensionDeclSyntax nodes using that exact type syntax.)