The main ones that I'm still waiting to see integrated into mainstream Linux twms are workspace scrolling (of course), dynamic layout rules and dynamic offset rules (important for ultrawide monitor users).

I'd also like to see container limit rules to enforce stacking after meeting a threshold (functions as a hard cap on tiles-per-workspace), and native support for Vimium-style shortcuts for every UI element on the screen (from jwno), but I could probably live without these.

I wouldn't call these particularly innovative features, in fact they are pretty low hanging fruit.