Along the lines of what the new Drupal 7 has done - out of the box and by default you get a plain text editor but installing the 'Wysiwyg' module lets you plug in any editor/s of your choice and configure them via the admin cp.
XF could remove the current built-in editor from core and replace with hook/s. Then convert the current built-in editor into a 'core add-on' or build the entire plug-n-play add-on or let third-party contributors build it.
I dare say its a big ask but a feature such as this might mean less threads about 'the XenForo editor' in the future.
(Apologies if somebody has already suggested this, I didn't find it.)