Sand Worms
They dig into the sand for cover and come up for to attack the player when wandering the deserts
when they are digging (a form of transport) they leave a trail (the footsteps that players make)
they can only travel through sand blocks
Reason and why it is good:
It Populates deserts and sand biomes

Sand Boss
Spawns in large open pits when you walk over them you fall in a large pit . This Pit then gives you a little bit of space between the Boss and the player. as an Addition to The Wither and The Dragon Boss . Reason why it is good:
This gives you a Overworld boss to come across every now and then.

This would improve the endless walking or running through the harsh deserts and give the player more of a challenge . and + it would add an extra few mobs to the game (they could also in addition make scarabs that hang around dark sand areas

And Another Possible Creation Could be Monkeys and bananas the populate the Jungle biomes aswell as Ocelots - Monkeys through dirt blocks or even bananas or Extriments

Make Sure to Check out http://www.planetminecraft.com/project/eltharin-below-zero/