If you have a decent web browser, spell check is already there (By decent, I mean something other than IE)
I like the idea, but add an HTML button so you can do some neat effects and stuff :p
and along with adding Decals to the message, you should be able to attach models and decals to the message for the other person to take (This would mean the person automatically has the model/decal in their inventory, even if it's not free, so you could give people gifts without other people getting them)
I like the idea of adding things to messages, they're too bland :p
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