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

Field projections (Tracking Issue for Field Projections · Issue #145383 · rust-lang/rust · GitHub) are likely to need new syntax, although I don't think the specific new syntax has been decided on yet. Pattern types (Tracking Issue for pattern types · Issue #123646 · rust-lang/rust · GitHub) are a new sort of type which will need some syntax. For the time being (while the feature is being developed), they're using a macro-like syntax, but the macro will need to expand to something and it's plausible that a more built-in syntax will need to be adopted.