TES aims to be an open world RPG, but Bethesda seems to be working more towards a cinematic and railroaded narrative structure.

In order for TES to survive and remain a sandbox game, I think Bethesda needs to rethink the way they approach developing their games. I think this lies in greatly expanding all of the main four guilds, i.e. Dark Brotherhood, Thieves, Mages, and Fighters. My idea might be somewhat complex and I might have a hard time getting it across but here goes.

Instead of having some tired chosen one plot where you can kill some god or save the world or whatever, your character can work his or her way up in a guild. However, the difference here will be that becoming a member of a guild will be much stricter and more difficult and working your way up will involve politics and the right skills. You may not even be able to become the leader, or even want to. You might become the right hand man or just the leader of one of the chapters in one of the cities. What this means is that if you do become the head of one of the chapters you will be able to take on what jobs you want, pick who gets accepted and maybe even change policies, etc.

Along with that, the player will only be able to become a member of one of the guilds, but because of the level of involvement that is required in each guild this will not be a problem because there will be almost infinite content. This will be achieved through evolving storylines that are affected by what the other guilds do and what you do and in what guild you do it. This will create a series of interrelated stories and dramas that will play out differently every time someone plays a new game.

Along with deciding contracts and the like, you will be able to partake in missions if you so choose. You will also be able to determine the future of the guild you are in and may work with other guilds depending on the circumstances.

There could also be some kind of reputation system that you have and that your guild has with the various towns, races, and people of the province.

You could also build new chapters or additions to the guild, etc.

The vanilla game will involve multiple plots and possible outcomes involving the guilds, how they interact with each other and what part they play in each plot. Expansions would involve multiple more plots and, perhaps, crises that occur in whatever province the next game takes place. Even more so, the guilds would have to deal with some issues pertaining to happenings in other provinces.

This might be a bit ambitious or even impossible, but I think this would be great.

Also: Skills and attributes
For example, if one of your major skills is Mercantile, your character might be more of a merchant or some kind of arms dealer. If one of your major skills is speechcraft you might be better at politics and influencing people, etc.