I really want Mojang to do something special with hay bales. I was thinking that, when placed as a block, it would heal any nearby farm animals (horses, cows, chickens, etc.) with no visible effect, allowing you to make up for having to occassionally punch an animal to get it off of a fence or anything else of that nature. Another example: your horse has gone on quite a journey with you and so has taken a signifigant amount of fall damage. Then, instead of feeding it hay, you could put it in a stable stocked with hay bales that will slowly heal your trusty steed. Like I said, it might be better if there was no visible representation of this effect, but that's just me. If it is found that a heart particle effect (similar to love mode hearts) is neccessary, then so be it. Another possible effect is that it would "fatten" any nearby food source animals so that they would produce more drops when slaughtered (and chickens would lay eggs more often) or possibly make baby animals grow up faster. I would love it if Mojang would do these things. They just seem like good ideas.
Now they need to make them less suicidal (from 4 block heights or so).
I also find them in a "When danger's near or feeling doubt, run in circles scream and shout" mode every so often. Usually it's indoors over a staircase, but I've seen more than one of them sprinting in panicked circles out in the wheat.