wardens have much more diverse gameplay than vanguards

and they didn't need a class update to achieve it

have much more variance than -- and i don't mean just by mechanics

cross between a tank and a mage, with anti-mage shields and high poke + cross map follow-up anti-mobility beast, one of the best disengage spells in the game fighter/tank hybrid, has an anti-AA zone and global protection making him a uniquely good splitpushing tank has one of THE best peel spells in the game, gives an ally a map-jump with him for flanking.

the biggest thing that to me makes these guys unique is that they are made to counter other champions. Poppy shuts down jumpy champions, Galio beats mages, Tahm counters burst and pick champions, and Shen is excellent into AA-focused fighters and marksmen. The initiating tanks on the other hand are as generally good as can be; they throw their CC indiscriminately and they'll pretty much just relentlessly spam their spells while in the middle of a fight. The closest thing they have to that is Maokai's passive punishing casters for spamming on him, but that's really more of just an add-on; it's not really a defining part of his kit nor is it usually a compelling reason to pick him (maybe that's just because he's overpowered at the moment, though)

funny enough I thought that the vanguards were more diverse than before when they weren't interchangeable CC machines. No other champion had the ability to absorb team damage like Maokai*, Zac was super-disruptive with his 3 bounce ult and his long range jump, and Sejuani scaled into a late-game teamfighting beast with ranged initiation. Lack of niche was not these guys' problems. Post-update the biggest difference between these champions is the type of CC they're perma-CCing you with, and it's a real shame.

* I get that Maokai was reworked specifically because of his ult, but really, all they needed to do was either revert it back to before the FIRST rework when Maokai had to place his ult in a specific spot instead of having it attached to him, OR condense the effect into a short-duration absorption to solidify him as a strong anti-wombo tank. I don't find his new ult to be interesting in the least and I certainly don't think he's any healthier of a champion.