1) Ability to view system folders (inbox/sent/drafts/deleted) as well as create your own for archiving messages
2) Save templates (i.e. save x-mas wishes, or birthday wishes) and retrieve easily to reuse without much fuss. It would be also very good if you could backup those so that you can restore when you hard-reset.
3) A very nice feature will be the ability to resolve a phone number within a message to the contacts name. For example, when you receive a notification from your carrier about a missed call, instead of calling that number in order to find out who it is, it would be nic to see the name directly.
4) Also, it would be nice if the sms app would wake up the display and if the led could flash forever until you confirm the notification.
If anyone found something that covers those requirements, I will owe a big "Thanks"