The only card that hasnt popped up some other way is that original Rarity Bronycon card.
Technically it has. There was a flash sale for the series 1 Rainbow Dash promo and they had more orders placed for her than they could fulfill, so they took all of the names of the people who had placed and order and did a random drawing of 25 names to receive series 1 Rarity instead of RD, as an apology. I was one of those people. Then they had another flash sale for Rainbow Dash a little while later that I was able to snatch up to complete my set.
Unfortunately when they do a "pick any card" flash sale, Rarity and Rainbow Dash are excluded.
They had mentioned a flash sale for the series 2 Smarty Pants promo a while back, but never went through with it, which is why I think that's probably the best card to go with. It was only given out at a con earlier this year, if I'm correct.
As you said though, it's always more about what the person wants to add to their collection.