It's slow.
Pretty much all the new characters will be event ninja, not counting the ones from Sage treasure.
When they update the shops they will add Konan, Itachi, and two of the pains to the shops. So instead of adding new ninja you just reuse the old ones. Everyone pays for the ninja from Sage treasure instead and farms the Sannin from 60 elite instances or the Akatsuki from shops (they all become 50% off).
Then when they update the shop again it's to add people like Kakashi and Asuma, the special versions, plus more pains and special event characters. Event characters themselves become temporary obtainment for a long time and you have to spend to get them usually. So new ninja are not going to be available to free players for a long time.
Third Hokage is probably the next ninja that will be available but you c*ready see he's in the game. So what you are looking for is probably ninja that aren't in the game already and that probably won't be for another year.