TL;DR List

• Parent queries. As in, selecting an element any-which-way, then selecting the parent of that element. We have some proof it's possible with :focus-within.
• Container queries. Select a particular element when the element itself is under certain conditions.
• Standardized styling of form elements.
• Has/Contains Selectors.
• Transitions to auto dimensions.
• Fixed up handling of viewport units.

Jen Simmons asked what's on our lists

Making a wish list of all the things I’d love to see happen in the world of CSS in 2019. What’s on your list? Things you want to learn? Problems you want help solving? New properties you need? Design ideas you wish you could code? Resources you’d love to have? Name your desire.

— Jen Simmons (@jensimmons) November 14, 2018

• Aspect ratios (it's weirdly tricky in CSS, coming to HTML probably, and maybe we'll get it natively in CSS with a property someday)
• Exclusions
• Regions
• Subgrid (it's coming!)
• text-wrap: avoid-widows-and-orphans
• Nesting
• Outline with radius
• Animated background gradients (without faking it by moving them or whatever else)
• text-overflow:ellipsis over multiple lines / line clamping</li>
<li>0-to-auto transitions</li>
<li>Parent selectors</li>
<li>Async <code>@import
• Math functions like sqrt(), sin() and cos()
• font-min-size and font-max-size
• Masonry

Tab Atkins wanted to know what parts of CSS give us the most trouble

What parts of CSS give you the most trouble due to browser behavior differences? Collecting some data, please RT. (If answer is different for mobile vs desktop, let me know.)

— 💖Taudry Hepburn💖 (@tabatkins) October 3, 2018

• Tons and tons of requests for a standardized way to style form elements — not just for styling desire, but for accessibility to prevent trading standards for styling.
• Being able to test browser support of more than just property/values with @supports
• Improved handling of viewport units and their relationship to other browser UI
• Improved handling of multi-column layout handling
• Elastic scrolling

Tommy Hodgins did a CSS+JS advent calendar on Twitter that documents interesting possibilities

Dec 1: Parent Selector 🎄🎁 Though CSS doesn't have a :parent selector, you can create your own with a small JavaScript function and use a selector like [--parent] in your CSS stylesheets today!

demo: https://t.co/9N1A1Un8XT
code: https://t.co/NNUuvOi1zH#css #javascript pic.twitter.com/Nv8V3rl2AF

— Tommy Hodgins (@innovati) December 1, 2018

• Parent selector
• Has selector
• Closest selector
• First in document (like how querySelector works)
• Elder sibling selector
• Previous sibling selector
• Contains selector
• Regex selector
• Computed style selector
• :nth-letter / :nth-word
• Media pseudo selectors
• Not-blank valid/invalid selector
• Element queries
• Attribute comparison selectors
• Custom specificity
• Visibility selectors
• Overflow detection selector
• User agent detection selector
• Storage queries
• Date queries
• Protocol queries
• Deep hover

We asked as well

Putting together a list of most-wanted native CSS features. There are lots of ideas out there!

Hit me with quick hit ideas. Don't think too hard.

— CSS-Tricks (@css) December 17, 2018

• Container queries
• Has selector
• Regions
• Color modification functions
• Nesting
• Shaders
• Transition to auto dimensions / transition from display: none;
• Previous sibling selector
• font-size: fit;
• Styling grid-template-areas
• Animation between grid-template-areas
• Types for custom properties
• clip-path accepting paths
• inline-styles: ignore;
• Aspect ratios
• Scoping
• // single line comments
• Corner shapes