Just considering attack and Hp, how much looseness can you allow before drawing the line that signifies the end of a niche?
There are 4 stages
Need: The element needs something in this slot.
Value: The element would value something in this slot
Neutral: The element doesn't care. What else does it bring to the element?
Harmful: The element would be harmed by the addition. What else does it bring that justifies this imposition on existing cards?
These stages do not have constant ranges and are influenced by existing cards however I will give a rough estimate.
Need: Nothing within +4/-4 attack
Value: Nothing within +2/-2 attack
Neutral: Nothing with identical attack and within hp range
Harmful: Same attack and hp range
So on one hand if there is a gap of 5 attack (ex. no creatures with 3,4,5,6 or 7 attack in the element) then there is a niche seeking a creature (with 5 attack).
On the other hand if a card has an ability that would be valuable to the game it may be able to justify having the exact same stats as an existing creature.