I think a tenacity drake doesn't really fulfill any new sort of niche for a team.
Fire drake gives your team more raw power, it's the teamfight drake.
Earth drake gives your team bonus damage on objectives, it's the turret taking and baron/dragon control drake (especially due to augmented smites)
Water drake gives you huge amounts of sustain, it's the siege drake (and the laning drake, it's definitely most important early game)
Air drake gives you out of combat movespeed, it's the rotation drake
Every different elemental buff augments something that provides benefits for a different aspect of the game, teamfights, objective control, sieging, roaming and rotations. A tenacity buff would just be another teamfighting bonus, overlapping with the AD+AP buff from fire drake, while also making you slightly more difficult to lock down, overlapping with the roaming potential of air drake.
I think instead a new drake should be thought of from the top down, what's an aspect of the game that none of the other dragons currently cover? (I would say splitpushing but that's typically been Herald's domain).