Hiya,
I did a quick scan through the first few pages of the list (checking out those suggestions that have 100+ votes), and this is what I noticed (numbers are current rankings, but shouldn't change much):
#3 - Workshop material selection -> we can do this with dedicated stockpiles now, should be cleared of votes
#9 - Multithreading support -> will NEVER happen (from ToadyOne himself), should be cleared with an appropriate note telling people not to vote for it
#10 - Adventure Mode Skills -> most of the listed ones have been implemented, with more listed on the development page, so can probably be cleared
#23 - Retire a fortress -> implemented, needs to be closed and labeled as such
There are quite a few more suggestions below the 100-vote cut-off that I used which also need to be cleared, but for the most part they have less than 10, since they were apparently already cleared at some point. Clearing the four I've singled out would already allow a number of folks to reallocate at least one of their votes to something else (and allow 3 suggestions to move into the top 10, possibly the following: Farming Improvements, Realistic Mining and Guilds).
So, I'm in favor of someone (I hesitate to name Toady or Threetoe, given that they have so much else to do, though they would have the best idea for what to remove as [DONE] and have the least likelihood of spawning arguments) going through the list with each major release and marking items as either "implemented" or "worked on" and clearing the votes from those specifically, but not the others. If there is an announcement (e.g. on the devlog), then players could then go back and re-evaluate/re-assign their votes accordingly. I think that was originally the intent behind the eternal voting system.
-Dame de la Licorne