But I say go one step further - if some players like to be reminded of their place in the world, then add a few credible opponents for this as well, which are so way above your head that defeating them is not in the realm of possibility anymore. They can be encountered but require that the player adds a few rational skills "behaving unprovocatively, bribe or avoid" to his skillset (crush or crush later). Otherwise enjoy all hell being let loose - on yourself. And the best reward - if any - being to survive and escape without losing more than an arm and a leg.
Please discuss.
Making an undefeatable opponent defeats the purpose of making it your job to complete the main quest. No matter how selfish this undefeatable enemy is, they will still do what you aren't powerful enough to do if it has bad implications for them.
Let me tell you about a game called Spore; specifically the creature stage: In the creature stage, there are things known as "epic creatures" which are gigantic creatures with 1000 hitpoints, a native regeneration ability and enough damage to kill any creature not purpose-built for killing an epic in one hit. Getting within its attack range is not a feasible means of attack and if it can attack at range then your creature may as well just drown itself because you're not going to kill it. Ever.
But do you want to know the good thing about it? It's still accomplishable, and it can not be done alone (well, on a technicality it can, but only if you can attack at range, have at least half an hour that you can't find anything better to do with and if the epic itself is very slow). Even when you do have a group of other creatures with you, it will still involve running by and sacrificing nearby nests of other creatures to keep it distracted long enough for you to create some distance between your group and it and if you die then by the time you assemble another group and find it again it will be back at full hitpoints, and you will not be any stronger. In fact if your previous group died, chances are you sacrificed the strongest non-hostile creatures at your disposal, the only other ones you could not defeat alone, never to see them again, therefore making your group weaker as you will have to fall back on creatures you could defeat.
So allow me to make a better suggestion:
Opponents who are meant to be faced with a group. Ones who a player will, inevitably, have to flee from upon the first encounter, but should, shortly before the end of the main quest, be ready to face them and have a group willing to help them do so.
Alduin, if you fight him, should be fought alone, with some divine power lent to you so that you can do so which fades after you defeat Alduin or something like that, just so for once you can defeat the god at the end of the game yourself but still not be a god yourself.
As for the demands to fight multiple dragons, I'm sure that some of those who suggested it did so because they might be thinking "at some stage I will be able to kill dragons. I shouldn't end up being able to kill ten in a row because rather than all chase me at once they decided to form an orderly line and fight me one-by-one". In Fallout 3, I needed to enlist some aid in order to get into Vault 81 because most of the enemies inside were super mutant overlords with tri-beam laser rifles and more than once I'd find myself fighting at least two at the same time. I think it stands to reason that sometimes dragons should attack in large enough numbers if need be to force you to flee.