Morrowind wasn't like that, in Morrowind, you were the hero, in Morrowind, you were the Nerevarine, the prophecized reincarnation of the ancient hero known as Nerevar who came to save the day. Now, there's been debate as to whether you're actually Nerevar Reborn or just a pawn made to fulfill the prophecy to suit Azura's purposes, but that's beside the point, because one way or another, defeating Dagoth Ur was
your job, you weren't just there to help the real hero, so no, it's not really common among the Elder Scrolls series for you to be the hero's helper rather then the actual hero. And we must remember that in Oblivion, it actually sort of made sense, Martin was the charater who was special, being the last heir of Uriel Septim, you were just a random prisoner. While the Emperor does suggest that destiny somehow chose you, since it's never made clear and really doesn't affect anything, I don't pay much attention to that. In Morrowind, the prophecy played a central role in the story, in Oblivion, the Emperor's talk about how "Only you can stand against the Prince of Destruction" and all that was just kind of there, and I never really felt that I did anything that anyone who was skilled enough couldn't do, by comparison, what Martin did was undeniably something only he could do, in this case, it did sort of fit in with the story. It still kind of annoyed me, in part because the conclusion was somewhat underwhelming, and Bethesda did not have to write the story the way they did, but regardless, Skyrim, it seems, will be different. You are the Dragonborn, and everything we've heard paints your character to be some sort of impressive hero, the last Dragonborn and the only one who can stop Alduin. What this tells me is that Bethesda has decided to go back to you being the chosen one. o longer are you just some random prisoner who just happens to be in the right place at the right time, you're the hero because you're the only one who can do what you must do.
Isn't like 90% of the endings of Daggerfall that someone else gets the Numidium and uses it, with you in the sidelines and the one ending where you control the Numidium you get squished. So end-game you not being the total hero isn't unique to Oblivion. And in every Elder Scrolls game you have been working for someone besides yourself.
In that case, though, it's a bit different, as it's not about saving the world, which the story was in Oblivion, it's about helping whichever side your loyal to gain power. The ending if you choose to control the Numidium yourself seems like kind of a jerk move on the developer's part, though. It's like the developers saying "You just got the artifact of ultimate power and want to use it for your own purposes? Well screw you! You die a horrible death instead!" But regardless, it makes sense that it would be someone else who uses the big stompy robot because, well, you let someone else have it. It's not like in Oblivion where sitting back and watching Martin summon a deus ex machina is the only option, and not because of choices made by your character, even ones you have no control over.
I thought the ending of Oblivion worked pretty good; I figured Mankar Camoran was meant to be the final boss and the stuff after that was the post-game epilogue. Maybe it could've helped to give Camoran a more unique appearance so he'd stand out more as the final boss.
I'd definitely say Mankar Camoran was the real final boss of Oblivion, seeing as you actually got to fight him, and that's part of the problem, because as final bosses go, he was pretty underwhelming. Admitably, Dagoth Ur was too, but I still consider him a step above Mankar Camoran as a boss (I'm talking just as a boss fight here, how he stands as a character is a subject for a different discussion.) because he actually looked unique, not very impressive, but unique. Mankar Camoran just looked like any Altmer in a robe, but Dagoth Ur wasn't quite like any other foe you'd face, there were also special steps you have to go through to defeat him, rather than just hitting him until he dies.