Now, see, THAT is a well-thought-out and well-said argument for what is wrong with hard modes, not in the context of "they are broken and should be avoided in the future" but "they are broken and should be improved in the future." And I agree with you that this is what the OP was trying to say. Actually, I think you make a really good point. We've seen something similar to what you suggest way back in ZG in Vanilla WoW, of course; the more primal gods you left up, the more different attacks Hakkar had at the end of the instance.

The fact that the only hard mode we know of so far in Ulduar (Flame Leviathan's "defense towers") sounds suspiciously similar to Sarth and his drakes worries me. Though, hope springs eternal: Since he's part of the "V0-L7R-0N" defense platform, maybe he combines with the undestroyed towers in some way, making him tougher the more of them he has to feed off of? That would be better than "fight the boss and the towers at once."

If we're lucky, though, somewhere in those 11 bosses in Ulduar we'll find some new mechanics for making hard modes harder. That or Blizz will have to face a lot MORE complaining that it's boring and hard modes are becoming gimmicky.

Simple solution to your problem.
If you want the hard mode fight to feel fresh and new, just don't do the easy version.

If hardcore raiders are so hardcore and awesome at raiding they should just do it hard mode from day 1.

Then, the content that you say if just recycled will become something you never seen before.
And since you don't know how the easy version goes you'll get a hell of a chalenge. You'll love it, amirite?