Perhaps occasionally there could be skins which cost a VERY high amount of cookies.
People who have been playing the game since launch have amassed hundreds of thousands, if not over a million cookies.
These cookies are useless. They're just sitting there.
The shop could feature a skin for 250,000, 500,000, 750,000 or 1,000,000 cookies occasionally.
A fun reward for people who've stuck around.
Cheers!