A rarity factor could be nice, if its fully pointed out that aside from the regular figures, there is a rare one that occasionly pops up.
That way you wouldnt expect the figure to show up and you can get pleasantly surprised. Later on the rare figure could be released through DS once he is out of stores, to prevent crazy second hand prices for it, and would give a nice period of rarity before being made available to all.
Perhaps there could be two rare figures at any time: a male and a female one. Could be just one of them in a full box.
The two could match well together, always being two figures of the same theme with each batch.
Like a king and queen, a male and female ghost, a gentleman with a powdered wig and a lady with elaborate dress and a high wig, etc.
Maybe for one batch of surprise figures, kids could be used instead?