You won't be able to change the mouse icon atm according to Narrel.
I really look forward to this! Although the functionality so far is quite limited, what Narrel said is currently included in the API satisfy a lot of the basic things I would want to modify/fix/unbreak. And it can only improve, probably...
I really love Lua as a language, it has all the good stuff
. I really look forward to start writing addons again.
What I really hope they open up to the API is the combat log. Fine, they don't have to include all the event info WoW has if they don't want us to make boss mods. But, the current state of the combat log is appalling, it's hardly readable, and contains virtually no useful information. With a proper API to it we could clean up the output to actually be useful/readable. And we could make a full HUD.
And we could most likely borrow a lot of code from WoW addons, just replace the proprietary API calls with new ones.