Start with all of the known safe cases (basic types should be fine), then move on to more dubious options (anything that supports iteration). Then allow iterable types but don't allow iterating over a mutable reference. And so on. If it's a priority to loosen up the rules without sacrificing safety, surely some solutions could be found to improve ergonomics.

If you want guaranteed safety then the borrowing rules are the most flexible as far as we know.

Just to give a couple of examples of how your idea might be flawed, what do you consider "basic types"? Are enums basic types? Then you've got an issue, because you might get a reference to the contents of an enum and then replace the enum with another variant, and suddently you've got a dangling reference. Would you prefer to prevent creating references to the contents of an enum? Then you're more restricting than the borrowing rules.

Allowing iterable types but not iterating over mutable references is not enough for safety unfortunately. The basic example is getting a reference to an element of a Vec and then calling push on the Vec. This seems very innocent, but pushing on a Vec might reallocate its backing buffer, making the previous reference dangling. Again, would you prevent taking references to elements of a Vec? Then again you become much more restricting than the borrowing rules.

I really liked the idea of an optional, built-in GC w/ pre-1.0 Rust where specific references could be GC'd

That was just syntax sugar for Rc/Arc, and you can still use them in today's Rust, albeit with slightly worse ergonomics (no autoclone for example).