As a costless, colorless card, Malignant Cell as a standalone would be hideously overpowered. A 30-card deck with 6 of them could expect to have a full field by turn 4 if they slapped down 3 of them on the first turn. It's hopelessly abusable:
Mark of Fire
8x Emerald Tower
2x Burning Tower
6x Feral Bond
6x Explosion
6x Malignant Cell
2x Firestorm
...no direct damage to you, and you're healing 46+ HP/turn and dealing 23 dmg/turn by turn 6 or 7, with Explosions to counter enemy shields. Virtually the only defense is Emerald/Jade/Mirror Shields, which are rare because they're not that useful.
No, Malignant Cell will never be a standalone card.