Where to start?

The initial challenge is to set up the general setting of your plot and since Mundus and Oblivion are huge and their history stretches back a few millennia there's a lot to choose from. Luckily I had a general idea of what I wanted to weave together and even tested it out as forum roleplay for a bit before starting this campaign. And the nature of my group of "pets" and what they wanted to experience helped narrowing down the list of choices.

Where?

I had no plan to limit the campaign to a specific area of Tamriel and I want my players to have the freedom to choose their own direction. But some areas are more or less interesting in my opinion. Skyrim for starters is an area I want to avoid, 'why?' you ask and my reasoning is quite simple - Skyrim is set in Skyrim. One of the conditions I had for my players was if they had or could buy the game and play it to get more immersed with the Elder Scrolls lore which means all of us had spent hours exploring the land of the nords and it wouldn't be new and exciting as a setting. Besides we all live in the northern parts of Sweden and we could just walk out the door for that kind of nature.

But I'm quite sure they will head to Skyrim at some point because almost all the player's characters have a connection to that part of the world for one reason or another from being born there, having fled there from Morrowind or even just visited the area for the wildlife.

As for the beginning of the campaign, I set it in Cyrodiil because I felt it was the most likely place these character would cross paths. More specifically it all began just outside the city Bravil for reasons that are more tied to the initial plot and to where I wanted to steer the campaign to start with, Elsweyr! You will come to learn that khajiit are close to my heart and that's why I wanted it to be the first land the players visited. Now someone is going "But you wrote that you wanted the players to be free to move about as they wanted!" and I do but it's not a good starting point because if they were allowed to sandbox right from the start, then I'd have no way of planning out the central plot. Before I gave them more or less complete freedom of action and direction, I wanted to start up the main plot as well as do an Elder Scroll classic. *wink wink*

When?

The obvious choice for a lot of you might be to just set the campaign concurrent with the events of Skyrim, but not for me and here's some of my reasons why:
We know pretty well what goes on in Skyrim around the emergence of the dovahkiin but we know very little about the current state of the rest of the world and what we do know is kind of old news. For example we are told about the situation in Hammerfell but it's more or less old news since what goes down happens 25 years prior to the appearance of Alduin. And that's an issue when the roleplay takes place outside of Skyrim but could also be problematic if they were in Skyrim since there's a lot of alternate paths for the dragonborn which shapes the setting.

Instead I've chosen the year 171 of the Fourth Era. It is the year when the Great War erupts between the Aldmeri Dominion and the Empire, an event that I wanted the players the be caught up in even though I'm under no illusion thinking that they could actually have a huge impact on the war itself.

It also takes place after all the major and vital events, or at least he once I consider important such as the Void Nights and the Red Year and invasion of Morrowind. Most events that takes place after the Great War are connected to Skyrim only and nothing I think has a huge impact on the setting, apart from Hammerfell leaving the Empire.

The biggest pro with setting the campaign sort of in the middle of history is that I got the final results, so to speak. I know what happens in during the war and what takes place a few years in the future and it works as really good limitations to create your plot within.

The Antagonist

On this topic I am not gonna be a hipster though because I cannot imagine a better antagonist than the Thalmor. A powerful faction with a mysterious past that exercise threat, extortion, torture and assassination among other things to further their rule and amount of power. Bethesda essentially created them to be the perfect enemy. But one of my goals in the campaign is to make the Thalmor "more human" because I think that in Skyrim they are portrayed very badly, there are still a lot of people that fight under their banner and not all of them can be blackmailed to do it, can they? To me that doesn't make sense.

And making their enemy a faction instead of a single person makes it a lot harder for the players to exact revenge on those who set them up and will make them act with stealth and cunning, which is the theme they chose themselves. But there will of course be other antagonists connected to other plots with no affiliation to the Aldmeri Dominion whatsoever.