I don't have any missed opportunities yet, the only part of the province I have yet to ride is west of Red Rock, lake of the woods is on the bucket list.
For years the D trail on the north shore of Superior had eluded us, it wasn't completely open for many years due to low snow conditions, in 2009 the stars all aligned and the trip was completed and it was an amazing ride. The following year much of that district was dismantled and the D trail has never been complete since, so that was almost a missed opportunity.
Organized snowmobiling has changed so much, I still ride but not on OFSC trails, gone back to the grass roots and it is so much more enjoyable. I think the haydays where 10y, 2000-2010, the organization was really rolling and for the most part riders where respectable, then the wheels came loose and things changed, the whole deal is off the rails now.