Ulduar was IMO the heyday of WoW. And although the Ulduar hard modes were obviously not going to work as the baseline (there's only like three ways to do it, adds like Freya, boss order like Council of Iron / Bug Trio, or timer like Hodir. IMO Firefighter was clicking a button inside the raid as opposed to a button outside where you choose your difficulty.) I don't see why 1-2 bosses per tier couldn't have such an option, especially on heroic difficulty. Especially if the boss itself was optional.
Nothing crazy, like a +3 ilvl upgrade or something. Could also help fill a niche for guilds good enough to crush heroic but have a roster too big or small to do mythic. I'm also thinking that the difficulty and loot table would be set up in such a way that it would not be the best way for mythic raiding guilds to spend their time.
It'd also mean the baseline encounter could be easier, which after the stratospheric difficulty of Sepulcher would be good.