Currently there are few ways to declare a dictionary-like type:

// Key can be only `string` and `number`
type _ = { [key: string]: string };
// Key is any type usable as object key
type _ = { [P in string]: string };
// Same as above
type _ = Record<string, string>;

I think it might make sense to implement it as 2 rules:

• Disallow object types (or interfaces) with a single index signature member and fix it to mapped type, or enforce the opposite: disallow mapped types of string or number with unused key.
• Disallow mapped types with unused key and fix it to Record. It also may be extended to enforcing usage of other builtin types, like Partial or Readonly. Not sure if there is a need to do the opposite, since it can be done with ban-types.