I find that Bethesda is missing out on several plausible and highly believable sub-games that they could make. It does not require that they be a part of the main series (I, II, III, IV, V). For instance, assume that they based a more centric game so to speak, featuring the aspects of just one part of the game. Now onto the basis of the idea which takes in part to what this wikia is about; Assassination. Within the backstory of TES, there are comments about people of great dexterity which could incorporate freerunning. As well there could be a few ways to go about using the inclusion of freerunning. There are two guilds I find that would be perfectly suited for this kind of game; The Morag Tong and The Dark Brotherhood.
Reasoning: Morag Tong
First off there is one particular image that comes to mind which would fit beautifully for an assassins creed-esque version of an Elder Scrolls game:
This is a concept artwork of a Morag Tong assassin, note how there are the two wrist attached 'spines' for stabbing. Within the ingame lore the wrist blades can be modified a bit to be included. Such as saying that they are magically created or even that the chitin on the wrists house a living organism which responds to the tensing (clenching) of muscles to extend on it's own. Such as certain types of plants in the game.
The Morag Tong unlike the Assassin's guild in AC, have their targets selected not by being Templars, but by being political figures who have had other political figures take "writs" out on them. Afterwards the successful assassination, a Tong is expected to turn themselves in immediately after an assassination; so as to absolve them of any long-term legal ramifications.
Also when it comes down to it, the Morag Tong are a very legitimate group that people will associate with. Such as in say Assassin's Creed 2's games where Ezio would go about renevating businesses and buildings.
Setting/Locations: Morag Tong
There are several plausible time-periods which could be a decent choice to go with. As well as several different parts of Tamriel.
At the very end of the 1st Era, the Morag Tong were responsible for the assassination of Reman III Cyrodiil, the emperor of Tamriel. As well as the ending of the Akaviri Potentate by the assassination of Savirien-Chorak in the 2nd Era 430.
Time periods after this would greatly limit the Morag Tong to their native Morrowind as they had been fully supressed in other areas. However even then Morrowind could provide plenty of areas which would fit perfectly.
Reasoning: Dark Brotherhood
One of my most favorite portions of Skyrim were the Dark Brotherhood missions, simply because of the pure insanity of Cicero. Generally the Brotherhood are very small in numbers and their targets are far more wider and varied in backgrounds. The darker color theme of their appearance is fantastic in my opinion for an assassin.
Setting/Locations: Dark Brotherhood
Their locations and settings can very greatly. As opposed to the Morag Tong with their official guild buildings, the Brotherhood is relegated to the shadows and outskirts of places. In the basement of an abandoned building in Oblivion and out in the country in Skyrim. However, unlike the Morag Tong, they were not chased out of the rest of Tamriel during the 2nd era, so their settings could be just about anywhere but Morrowind.