Having when editing 50% of the screen place occupied by the wiki source and 50% occupied by the preview could be cool. Especially if the preview is in a scrollable box, scrolled by default to the position of the cursor in the textarea and updated in realtime (This have to be optional since for slow computers it could be dangerous :D)
Even if the preview isn't perfect the fact that it could be "real time" and on the client side make it usefull i think.