I agree that it's probably way overdue that we have a new Defense Guardian or even having our old Defense Guardians buffed.
But i guess, the reason we may not be seeing more Defense heros (in general) is because of how powerful the defense stat is.
Defense in this game is pretty powerful in that it can block out every physical attack in a 1:1 ratio. Not only that they are also able to block out MAG damage in the same 1:1 ratio.
It is pretty OP in it's own right. If they released OP defense heros, it may tip the balance such that you will see even higher ATK numbers from future bosses and that any non-defense centric heroes (especially mages - they are already very paper currently, but it may make every hero who isn't defense become like them) will not be able to survive, i.e. major power creep.
Other games make defense worth less than ATK, but that also doesn't solve the entire problem.
HP heroes on the other hand are easier to balance because HP is a fixed value and whether you can survive a strong attack is easily balanced by how large your HP pool is.
So while i would like to see more unique useful Defense heroes, i understand why we may not be getting many of them.