Saving more than 35k coupons make no sense, there is not and there will never be an event where would be worth to spend the coupons in excess after the 35001st.

In theory konoha great tree is an event where you could spend this ammount and much more, depending how good actually top level becomes. Also konoha outlets doesn't seem to have cap limit on scratch cards and you could just make power items supply for a year worth of rebates. But thats just theory, in current state of those events its not worth it.