More customisable themes and tiles allowing for better and more unique personalisation of WP7
One thing that I feel is missing from WP7 that is a basic standard in most other systems is customisation. Currently the best you can do is the lock screen and tile colour.... even a personal ringtone based on an MP3 is a complex and round about task to do.
I think it would be great if MP3's on the phone could be set as ringtones with ease.
I think that the background behind the tiles should be customisable as well as tiles them selves.
For tile design I think it would be great if it had a combination of tile back grounds (from solid colours, Gradients, Textures or even images) The ability to set your whole start screen tiles into one image split across the tiles (I am aware of apps that do similar but they don't function well and only work on certain apps and only act as shortcuts which increase loading time) This should be apart of the OS. I also think the basic images (eg. the sms image, email image, phone etc) should have a selection of unique designs to choose from.