Plague is a pretty cool card when you have some way to survive while the enemy creatures die. So it's not like Rain of Fire that you can just put into any mono fire deck. Still, there are better choices if you want mass creature control, like Pandemonium.
Plague is also kind of terrible unupgraded, it is simply not worth the cost.
The best buff Plague (and all other infection effects) could receive is a change in when the damage takes place. Currently, the creature is damaged only after attacking; if the creature was damaged before attacking, it would tremendously help all infection effects become actually competitive and useful. You would take one less attack from infected creatures.