The only thing I would want would be something that changes a group of bricks to a larger brick of the same size.
Would be useful to get rid of spam.
I think the absolute best thing would be Day/Night cycle maps and Dynamic Lighting.
It's nice to dream.
Would be nice, but probably laggy.
Also, more clothes and being able to see others their decals and faces without having to download the package would be nice. I don't see the point of wearing custom faces and decals when nobody sees them...