Add a warning for new keyword used on types which are not IDisposable
The idea is that the new keyword provides valuable information, but only if you do not use it on all types.
When avoiding its usage, you get a visual clue as to instances of disposable types, as well as warnings if you bind them.
By making it a warning to use new unnecessarily, the compiler would effectively enforce a "best practice" with regards to IDisposable usage. It goes a long way today, but requires discipline to make it useful.
This would be especially helpful to people coming to F# from C#, as many immediately use new everywhere, and don't see this very nice safety benefit provided by the compiler.