There are already a number (12) of harcoded urls (with a bias towards germans :)) for the webservice. It would be very useful if the user could modify them and so have presets to quickly change the current url. This is in the spirit of firefox search engines. Also, it would be awesome if there were a way to easily select the current "engine" (instead of opening preferences and going to the web service tab, etc). A suggestion: shift-drag selection to the dict panel plugin shows a menu of the user "engines".
Probably because the original xfce4-dict author is a German ;-) I could imagine to change the buttons into a list of presets that can be modified to your liking. Personally, I do not have a need to change this because I do not even use this functionality. If you can provide a patch that implements this, I will happily merge this. I recommend you start with a mock-up so we can decide what it should look like. Reopen if you want to do that, I am closing this old bug for now.