I truly would like Mojang to add these variants into the game. It would open huge new possibilities for builders and resource pack makers. Give us entire new color palettes to create and decorate with. Feel free to suggest more.

(Polished) Diorite Stairs, Slabs, and Variants (Pillars/Chiseled)

(Polished) Granite Stairs, Slabs, and Variants (Pillars/Chiseled)

(Polished) Andesite Stairs, Slabs, and Variants (Pillars/Chiseled)

Prismarine Stairs, Slabs, and Variants (Pillars/Chiseled)

Stained Clay Stairs and Slabs

End Stone Stairs, Slabs, and Variants (bring back End Stone Brick recipe)

Wool Stairs and Slabs

Basically very (at least full) block should have a stairs + slab version by default. I know it is very frequent suggestion, but that does mean we want this really really much.