General improvements:
*Battery percentage in the notification bar so I don't have to swipe to check it
*Palm rejection, particularly useful as bezels on both phones and tablets get thinner, and Samsung and Nvidia (rumored) include styluses.
*SMS/MMS sync and backup
*Support for SMS/MMS mesaging from tablets and PCs synced to your phone - this is possible with Bobsled Messaging, Motorola Connect, iMessage(?), and LG's latest tablet.
*Multiple lockscreen widgets per page
*Flashlight app with lock screen widget, and possibly quick settings toggle
*Infinite scroll for homescreens (scrolling through homescreens to the left or right will eventually bring you all the way back to where you started)
*F2FS filesystem, as
Motorola implemented with the Moto X
*An app center which allows you to disable individual app permissions in one place, and perhaps a control panel with links to each app's settings menu.
Quick Settings improvements:
*Long press to toggle mobile data
*Long press brightness to toggle automatic brightness
*Add Background sync toggle
*Add GPS toggle
Power saving:
*Adjustable percentage at which power saving mode should be activated, which would optionally control brightness, mobile data, GPS, background sync, bluetooth, and Wi-Fi, and prompt to enable if you're actively using the device, so that it doesn't suddenly dim the screen or kill your data connection while you're using it.
*Optionally disable Wi-Fi automatically by location
*Choose which apps can use data while the display is off, like Sony's Battery Stamina