Well when the mod API comes out in 1.4 I think we should add a few extra mobs!
For a start I think we should have horses, a mob which can be controlled and runs 2x faster than the average user. Maybe horses could spawn rarely and cannot be bread, allowing to keep them rare.
Another mob would be a "giant bird" maybe for Goldy's, this mob would only spawn in the air and is very rare, this could be rideable so would be very nice to see!