So I'm still stuck on this "open-world sandbox RPG without a Main Quest" concept. I think it might possibly have some legs, an opportunity to do something really different with the medium. Most RPGs tend to take a bit of a movie-style approach to their design (in that playing is at least somewhat akin to watching a really long movie, with some sidequests thrown in as a regular distraction.) For a game that's going to be essentially a collection of sidequests (or if you're making one without a main quest, I guess they'd all just be considered "Quests,") maybe a more episodic approach would be interesting.
(Disclaimer: by "episodic," I
don't mean "Bethesda should make Fallout 4 one of those games where you buy tiny bits of the game for $10 a pop, with a new episode coming out every other month or so.) What I'm thinking of is a complete game that approaches it's design more as a TV series following multiple seasons, than a movie where it's really all just one long self-contained storyline.
Here's what I was thinking of: If we consider each (relatively) self-contained Quest as one episode in a series; then a string of them together could be considered a Season, complete with a Season Finale which will generally leave on a cliff-hanger and introduce slight changes to the paradigm, which will define what's going to happen in the next Season. All of the Quests could be divided into the various acts that would make up the game - at the start only a portion of them are available to the player. (Quests, that is - this is still an "open-world RPG" we're talking about, so there's no reason to arbitrarily bar the player from areas of the game's geography; and there's always plenty of dungeons in a Bethesda game, and all of those would be considered seperate from the Quests in the game.)
After playing through a certain amount of Quests, it would unlock a "Finale" sort of quest; which would roughly tie all of your adventures to that point together in some ingenious fashion, while slightly altering the world in some key manner. After that Finale Quest was finished, it would "reset" the world and open up a bevy of new quests. There'd be nothing keeping the player from holding off completing all of the possible Quests in a particular "season," but you wouldn't necessarily have to do them all in order to move on to the next one. (And it wouldn't necessarily have to always "wipe" all of the Quests you didn't happen to have finished, either.)
What this would serve as would be a useful way to allow the game to react to your character's decisions in meaningful ways. One thing that's often bugged me about RPGs in general is that there's often a sort of "hub" town that you tend to go revisit over and over again (in Fallout 3, that'd be Megaton or Tenpenny Tower, depending on where you house ended up being.) But after exploring the place once, and doing all of the quests there, no one really has anything left to offer you. The place gets a little repetitive once there's no more NPCs worth talking to. If the game sort of "updated" itself every few Quests, it could take into account the impact of the choices you made in those Quests - slightly changing the new ones to take into account the choices you've made, and allowing the NPCs to react to the same, as well.
Since it's an RPG without a Main Quest, this is something that could repeat itself ad infinitum (or at least until they stop adding DLC, of course.) Anyway, just an idea.