Flexbox is the modern way to lay out an interface, however it still needs vendor prefixes. At the moment I'm using:
TweenMax.set(element, {
display: "-webkit-flex"
});
... which supports Chrome, Safari and iOS, it would be great to have this work for all browsers by display: "flex".
Why don't I just do this in CSS you ask - I have a couple of plugins heavily utilizing Greensock (think carousels, tabs, etc.) that I need to be JS only, so defining the essential structural styles in code is necessary. For me flexbox is the best and cleanest way to layout a carousel or sliding tab content (using Draggable).
I imagine the infrastructure is there to accommodate this essential CSS property. It would be a great addition to your CSSPlugin.