TES is about free roaming and non-linear play style. By just furbishing the things you need for your quests and only making those things accessible you need (at a given time), you kill that concept... completely. TES is all about the extra's outside of the main quest lines.
Also, I don't know if the next game will feature the Daedra as heavily as TES 4. You should remember that that one was about the invasions, while in former games you never got to visit a daedric plane in the first place, so you are basically asking for a close continuation and expansion on the Oblivion crisis (which is supposed to be over, the gates between the worlds are shut - now it's time for the aftermath, without going back to these planes)
The Main Quest province would remain fully furbished, at least as much so as Cyrodil in Oblivion. The rest would just be addenda of a sort. . . intriguing addenda, providing a less restrictive feel, and a greater sense of worldliness. So it would still be extremely non linear. . . more so, if anything, than the more restrictive format of having provinces outside the main quest completely inaccessible. . . it seems to me that this goes much further in killing exploration. .. that go no further barrier and all. I should at least be able to veer off and visit the seat of The Morrowind Mages Guild etc. It would only add more travel options, so I find the logic of your contention faulty.
And no, I am not at all asking for an expansion on the Oblivion crisis. I said that the realms of Oblivion should all be open, in a limited way (limited in terms of the amount of the realm Explorable, each one roughly the size of ONE of Mehrunes Dagon's Deadland variants in Oblivion, but encompassing the primary dwelling of The Daedric Prince, as oppossed to the more sprawling and memory consuming Shivering Isles. ) This would require nothing akin to the Oblivion crisis. Wizards have found ways of contacting Daedric Princes and visiting their realms for the purposes of study etc. without any such crisis or even the threat of invasion being involved. That is what I am talking about. . . that at a certain level, quests with great potential benefits to stats and attributes would open up involving The Daedric Princes, and that you would actually get to enter their realms in the process (think the gates that Boethiah and Peryite open for you in Oblivion, both of which could be accessed AFTER the Oblivion crisis was ended, depending on how you played. . . or The Shivering Isles situation, but again, in a manner not as expansive as the shivering Isles. . . and obviously, while you would be going in to meet and complete a task for the Daedric Princes involved, you would NOT be receiving grooming to mantle their roles). So no, it really would have nothing to do with the Oblivion crisis. Books of visitations by powerful Wizards and the like to the realm of Oblivion far predate the crises, and other voyages to Oblivion occur after it is ended. It does not matter that the PC did not get to visit The Daedric planes in former games. . . what matters is the precedent of NPCs in the lore being able to visit, and the fact that such visitations are possible, without anything akin to an Oblivion invasion being part of the equation. I am really only asking that more of the Mythic and Metaphysical aspects of the game be included. THis would be much more akin to an advanced version of the statue related Daedric Quests, than to the Oblivion Crisis. I will speak more on it in suggestions for Elder V, but I just wanted to clarify the point with you, in the event that you were confused about my meaning.