Just put all the available ninjas in the shops, except for the latest newly introduced ninjas, and let us eventually piece them little by little and update the shops every month.

I do not really undestand why is a problem if everybody is able to get the ninjas they want within 1-2 months WITHOUT USING THEIR COUPONS FOR THEM.

Whales would still go for them during the first month of their exclusive availability and 'unwise' players would still waste their coupons in wheels to get them earlier or to hope to get some good power up item, and whales would still spend on power to go ahead others.

Just let 2-3-4 ninjas whale only, like naruto ninetails chakra, itachi susano'o and edo hashirama, but after 1 month of their introduction put all the others in the shops. Otherwise just create new treasures and put a lot of them there.