The good arguments for "This should be a new language" are either:
1. You need to enforce a relatively unique set of invariants, which in turn come with new constructs to interact with them. For example, if you want a system for managing a distributed system, enforcing the lack of global state can be a lot more natural by using a language that cannot express it.
2. You want to provide some sort of novel language construct that cannot be expressed well in existing languages (function argument pattern matching, for example, cannot be retrofitted onto an existing language by a library, maybe by pre-proccessor or sufficiently advanced macro system).
1. You need to enforce a relatively unique set of invariants, which in turn come with new constructs to interact with them. For example, if you want a system for managing a distributed system, enforcing the lack of global state can be a lot more natural by using a language that cannot express it.
2. You want to provide some sort of novel language construct that cannot be expressed well in existing languages (function argument pattern matching, for example, cannot be retrofitted onto an existing language by a library, maybe by pre-proccessor or sufficiently advanced macro system).