yaml format can be removed, since it's now identical to json. format=txt and format=dump seem entirely pointless, and format=dbg seems redundant to format=php for real use and format=rawfm for debugging.
Now for the controversial part: format=xml seems to be a major source of problems, since it needs special handling all over the place. If we keep it at all, it would be very nice to change it to something that doesn't need magic "_element" and "*" members and won't cause bugs like bug 43221 (for the last, if nothing else define some sort of reversible encoding for invalid names). This would also allow us to get rid of format=rawfm, since we won't have any more magic elements.