A few simple suggestions

*A new game mode: mobs would be far more intelligent, forming large packs.

*You can recruit Iron golems (expensive!) to carry and place blocks. Since they move slow, they wouldn't be unfair.

*Structures need to be varied so that every experience isn't so similar.

*WE NEED MORE STURUCTERS! Travelling is pretty boring!

*Think you're invisible? Battle mode! Mobs form massive armies that attack every full moon! Skeletons horses and lots of armor and weapons.

EDIT: I expanded on my ideas in another post