@container queries are going to help a ton in making components more reusable - components that react to their own size rather than the browser size have been needed for a long time, and currently the only way to really do it is with the javascript observer api, which comes with huge performance tradeoffs.
Color-mix() is another item I've been wanting for some time, not because I need to mix things like red + blue, but because we've needed a native way to add opacity to css variables for a long time now. Having opacity be independent means we can have a robust design system that doesn't expand exponentially, while still providing opacity levels when needed.
Masonry layouts will greatly simplify a very common use case, color level 4 and 5 will make the web more dynamic and beautiful, and loosely typed properties will help add fallbacks. Definitely excited for these changes.