I think that GNU FDL is perfectly fine for Ultimate Wiktionary and there
is no need to change the license. The license is perfectly compatible
with the .DICT format, so there should be no problems at all.
Thinking about how to import data from wiktionary in the ultimate
wiktionary may pose a few puzzles with respect to FDL compliance, but I
don't see any significant problems. The import script should keep track
of who contributed to a chunk of data and take note of that fact. The
history may be a little more problematic, and I think we will want to
get advice on exactly how to do it.
But changing the license to something else would require throwing away
all existing work in wiktionary, which seems quite unwise to me.
--Jimbo