I'm going to go with the new one that is coming out Sunset Shimmer, glow in the dark Zecora and third will be Cupcake with reversed hair (blond first). This isn't included the already high priced ponies like Sweetie swirl and Star Swirl but only single pack ponies. Since most ponies that where bonus ponies will be hard to get to start.
I know I want a cupcake, sadly I had one but then didn't want it (sold it) then saw the reverse hair and was like that looks better. Also want Ploomette but I think she will be like Lily Blossom, BlossomForth, Diamond Rose, Plumsweet hard to get but rather common in comparison. I think a lot of the non mane six will stay popular and pricey.