Okay, first things first.
I don't exactly agree with you about the
**** thing. I mean, how many times will you replay the campaign? If you could beat the game in
**** doing all the things you could do(100% of the game), you could say "oh well, i've done everything i could do here" then pass to another game. Then, after some time has passed, when you're without that many game choices and with lots of free time, you replay it again. Well, that said, the campaign isn't complete, and I get you point of view. You're not wrong, but perphaps expecting so much of it?
We can only "give up" for now, but later(when the campaign is completed or almost) we will get "The End" to do too. I've heard of Aero some projects of doing bonus things(you could say, new game+ bonuses, if you know the term)for the ones who load a "The End"
Now, about the "Legendary Areas". Personally i thing that would be beyond difficult(perphaps impossible like you've said) to be made, but i like the idea and the concept. I mean, i would get very excited, that's for sure. I love new things that you discovery in a game when you think you already did everything(if the game is good enough, that is...)
Yet, i remember you that TCO isn't exactly a easy campaign. I mean, not all of us are such
**** gamers that can beat even the Touhou games or such
**** games with ease. Within a normal point of view, to beat later epic bosses in
**** can be ....
****.
"So Shino, are you saying that my idea is bad?" No, the idea is very good. Perphaps not a legendary area, but a alternative mode of the same map, with new monsters and stuff...that wouldn't be bad, indeed...Ahem! But there's a limit of how difficult a boss can be, because if it's too difficult, it's unbeatable, but if it's not difficult enough, the main concept breaks. Besides, your idea looks, like i've said already, difficult to be made, perphaps impossible.
Now just to end this post(it's getting too long, and i'm getting lazy), you should, at least, wait for the end of the campaign. It'll get more difficult. Have you seen the bosses that Aero will put there? They're
**** difficult to the point i've given up defeating then now(in the campaign i'll get revenge though). Well, perphaps you're even more of a
**** player than i thought and have already defeated them....Well, i can't help then. Too much hardcoreness for me to handle

.
PS: Doom, don't you think it is a little too unrealistic? I mean, is it even possible to do this? It would be interesting with it is though [-).