To me, the main story is not the most important part of any Elder Scrolls game. I mean, I've completed the main storylines of both Morrowind and Oblivion at least once, but I've also created many characters who never spoke to Caius or Jauffre, or who did do the first quest in the main quest but quit at some point during the proccess. The main story, in an Elder Scrolls game, is just one part that comes together to form a greater whole. For one thing, the main quest serves as a central questline tying the whole thing together, without it, the whole game would feel like a series of disjounted storylines which might as well exist independantly for all the connections they have. Of course, this doesn't always mean I want to do the main quest with every character, but I want to know it's there or the whole thing feels pointless. Moreover, the main quest is usually where the events that have the greatest impact on Tamriel's history happen, in Daggerfall, the main quest had you deciding how to handle the Numidium and who you should give it to, in Morrowind, the main quest had you killing Dagoth Ur, destroying the source of his powers and the Tribunal's, and becoming the Nerevarine. In Oblivion, you stopped the Oblivion Crisis at the cost of Martin's sacrifice. It could be argued that these were pretty major, world changing events. They're the things that really make a difference in the long run. I mean, sure, the Theives Guild quests would effect Thieves Guild members in the game, but it's not really going to change the ongoing history of the setting. And honestly, having all the important events take place off-screen would not be a good design choice. I mean, granted, things that are part of the backstory, of course, are shown in dialog and books, but that's because they're the backstory, but when the player doesn't have any chance to be involved in the actual main story, then you have a problem.
They also need to say in the games that several things are happening simultaneously, but you can't see them while you're directly busy with something else. In Oblivion, you're the only damn one leaving Cloud Ruler to get items, they have to start writing these main quests more as a team effort. You're really going to beg some dark elf nobles in Morrowind to let you go up the mountain by yourself and kill their greatest enemy? You're really going to waltz into Sancre Tor by yourself in Oblivion? There has to be someone else doing something for the cause. Anything.
You mean things like the Great Houses actually sending help for you when you attack Red Mountain? I'd say that would only be a good idea if they improved how the AI handles companions and large groups of people fighting at once. I mean, in Oblivion, we already had problems with that whn a few guards were fighting during the attack on Bruma if you had sought help from the other cities, imagine what it would be like if it was a larger group?
Still, when you said several things are happening simultaneously, what I actually thought of is that, if for the main quest, two or more tasks that need to be completed come up at the same time. For an example, let's say that we need someone to retrieve two separate artifacts. Whereas in Morrowind and Oblivion, the game would probably have forced you to retrieve both, the game could instead have the player choose which one to search for, and then have some other character be sent to retrieve the one the player chose not to look for. This way, the game could acknowledge that you're not the only one actually doing things, it would also add a bit of replay value to the main quest without the need to worry about how to reconcile conflicting outcomes as the same situation could lead to two different quests depending on which option you choose.
In Daggerfall you started with a shipwreck, and they didn't contact you to start the main quest till a month later in game time.
And I'm pretty sure I recall having wandered for quite some time in Daggerfall at the start before I find something worth doing.