"Quote Jeb- I've been working on making the biomes less flat. Also added a super-flat world type option: http://i.imgur.com/4ymFD.png "

The quote is what Jeb said on twitter.
It seems they are adding a new world type option that will make the world super flat if you select it.

And in the normal world it seems they will make less flat biomes appear.

This should be great because now that means there won't be as much flat lands like grasslands and more hills and possibly more mountains :D

As for the new world option that means people won't even post a flatmap in the projects section anymore.

They are also adding a new Mob AI system.
That must mean mobs will be more intelligent or move better. :)

They will also add modding support/Modding AI in the future....meaning modders may have an easier time making mods.

It also seems they are still thinking of adding a sky dimension to replace the original idea...though it might just be a world type.

They will be adding beaches.....again......as they were removed in 1.8.

This will be updated as more info is found about the future updates.