I don't know that I would say we want tanking niches. I ultimately am agreeing with what you're saying, but once we say "tanking niches" players have visions of the DK who parks outside of Icecrown until boss 4, 17 and 31 (yes, IC is that big).
31+ bosses? That would easily make the largest instance ever. My worry is are they putting quantity over quality? I'm led to believe that half the fights will be generic, recycled fights that we've already done. Either way, its clear as to why they added the raid lockout extension.
Other than Arthas, what are some of the possible boss encounters? I'm sure Sindragosa is one, as well as the Death knights Falric and Marwyn.
As for some speculation and a bit of lore discussion - how about another Kel'Thuzad return (Yes, again). The first time we brought his phylactery to Father Montoya who in turn returned Kel'Thuzad back to life. We didn't get a quest for his phylactery this time.
I would have speculated Anub'arak, however he appears to be the current last boss in ToC, and of course theres always the option of Bolvar Fordragon and Saurfang the Younger.