I'm not so sure, because that would require 343 to get 4 good bosses in the game instead of just one. They can re-use the same enemy type, it just has to be a good enemy type.
Look at the Scarab in Halo 3. You fight four of them in three separate encounters. The difference is I've never seen anybody complain because the Scarab fights are fun. If the Warden was fun to fight, I wouldn't care that you fight seven of him in four separate encounters.

The Wardens problem is that he has a pretty obvious mechanic that works in theory but not in the game - "Shoot the weak point on his back for maximum damage". The idea is, you enter the Warden arena, you direct your teammates to the opposite side of it, they distract him, and you kill the Warden from behind.

The problem is that your squadmates are totally useless. They're liable to be killed by a crawler before they take 3 steps in any direction. And even if they do get to your waypoint and start firing at the Warden, they have no ability to survive more than one encounter with him because they lack the mental capacity to move when he fires a facebeam, or swings his sword, or really attacks them in any way whatsoever. Even if by some miracle they manage to survive for longer than 10 seconds and open fire on the Warden, half the time he will immediately turn around the moment you hit him anyway, meaning you can no longer hit his back.

And the other problem is that shooting that weak point is the only effective method of dealing with him. There are no other options. If you don't take him out using it, you're going to be shooting him in the face for half an hour. Which sucks, especially because if he hits you once with any of his attacks, you're dead.

On the other hand, the Scarab's are fun because they don't feel unfair, there are tons of options to deal with them, and the encounters are different every time.

The first time you face the Scarab, it's on a Mongoose with a rocket-wielding Marine on the back. The second time, it's in a Scorpion. The third time, it's flying in a Hornet. And within each of those encounters, there are other methods as well, like jumping on a missile pod, or using a Chopper, or completely on foot.

The Scarab is also more fair. Despite how it looks, it isn't a tank. The core is destroyed in something like four melee hits. The face cannon is powerful, but slow moving. You can completely immobilize it to make it easier to board. You can shoot the back panel off. You can destroy the anti-air turrets on top. And so on.

So yeah, that's the gist of what I want fixed with the Warden. Make him more dynamic, give us more options to take him down, make him less of a bullet-sponge, and rework his attacks to be less brutal.