Don't know if this has been suggested already, but what about adding Ice Cube Baby, a new defensive baby that freezes enemies that touch him. It could also be use for a new mini game/challenge, Ice Cube Bowling, like the Wall-nut Bowling in PvZ, you place down the Ice Cube Baby that will slides towards the enemies and then bounce off in other angle possible hitting another enemy
Or what if:
1: it could throw itself and bounce off the enemies while slowing them and then respawn and wait a lil bit to throw again
2: it would act like the grape thingys from PvZ2 where it would explode into shards what whould bounce through the field hurting enemies but it would kill itself in the process.
and 3: it could be placed and you, the player/isaac could push it to hurt the enemies
The challenge seems cool btw.