An addition that i think would benefit most players is a wider variety of slabs, vertical slabs, and stairs for the majority of the blocks in Minecraft. I Believe that many players find it annoying that when building organic and very geometrical constructs, they cannot create gradual slopes depending on the blocks that they are using.

A Great use for vertical slabs would be creating steep slopes that could be used to create structures such as; Ramps in skate parks, obelisks in a graveyard or an ancient Egypt setting, better pillars, and many other things that i never thought of (But i mean, Lego never advertises every single structure that you could build right?). adding vertical slabs in addition to making slabs and stairs available to most blocks would greatly improve the minecraft experience for new and old players alike.

Now that we have focused on the "Vertical Slabs" part, now i will be focusing on the "Slabs and Stairs for All Blocks" bit.
With the addition of stairs and slabs for all blocks, generated terrain could look a lot smoother and less... everywhere. Even if Mojang never adds that to world generation, rest assured that someone will make an mcedit filter to make terrain smoother.

In conclusion, i think that adding this feature would make the minecraft experience a lot richer, but in adding it, it might push some veteran players such as 1.9 PVP did.

(Hmm.... maybe i am going to make a blog about 1.9 pvp...)