I took a look at all the issues labeled rust-2-breakage-wishlist to see if there were any good examples in there, but there weren’t.
Most of them are simply breaking changes to std, which the editions RFC specifically states cannot be done with editions (in fact, that part of the editions RFC uses https://github.com/rust-lang/rust/issues/35943 as its example). So there’s not much point gathering more examples like that.
The remainder aren’t great examples either for various reasons. One of them needs non-trivial design work so it’d probably go through a normal RFC if anyone did that work. One is simply incorrect. One is irrelevant now that ? works on Options. One would be a very controversial bikeshed. And finally there’s this minor issue involving CLI flags, which isn’t really what we’re looking for either.