Crowdsourcing request: What syntax changes that will affect the AST are currently in flight?

During RustWeek I had a chance to catch up with dtolnay about moving default_field_values forward towards stabilization. One of the (mild) blockers is incorporating support for it in syn. To minimize strife, the idea will be to release syn 3.0 with this change, but before we can commit fully to that release, it would be very useful to have a full list of all potential AST changes in the pipeline, so that we're not stuck with a syn 3.0 that doesn't support a feature a mere month after release.

We have an idea about making AST nodes non-exhaustive to better support evolution in the future, but the point still stands.

I am asking for help to compile this list. If there are any syntax visible changes that will likely require AST changes (lean towards assuming they will, if you're unsure), I would love to hear about them.