To be able to make music discs go into jukeboxes through hoppers and leave the box when they are done playing.
If you could clip minecarts together there would be a use for furnacecarts as an alternative to power rails there would also be a way using redstone to automatically clip / unclip them.
The ability for your dog to ride on your horse with you.
The ability to invite friends by typing their username into a multiplayer setting of some sort. It would be similar to pocket edition and would make playing with friends easier.
New limestone and basil blocks that are similar to quartz, they can be pillars or smoothed but also made to bricks chiseled and quartz would also be updated to be made for bricks.