More slab blocks
Slabs/Stairs that can be placed on the ceiling/upside down
Rugs
More craftable furniture
Diagonal-connecting fences/iron bars/glass panes
More mobs, both hostile and peaceful (and which DON'T alter the landscape)
Planting pots
More foliage (more flowers/plants/trees/etc. that have different benefits)
Climbable vines
Implement Zeppelin mod
(Mah ship shall sail!)
Ropes (like ladders, but can hang straight down, without toughing anything)
Alternate methods of creating power (waterwheels, windmills, solar panels, batteries, gas/oil pockets, etc)
Sheep regrowing wool >.<
Whales!
Selection of paintings to place, instead of randomized
Steeper stairs (stairs that are 2 blocks high, but can still be climbed without jumping)
Quivers
More weapons (Battleaxes, staff, crossbow, etc)
Shields
Customizable FLAGS.
Oh, I could go on and on in this one...