I think it matters what provinces we will see in the game. each has their own lore and bad guys in the provinces so where the province that the game is about would matter what we would see. I for one want a new bad guy who is trying to become immortal, and is like a demigod of some sort. like the Demigod son of Mehrune Dagon and he is like a badass battlemage and he is almost godlike but you're also gifted by the Aedra to defeat him something like that would just be great to see in a game like ES
More than an awesome boss, I would just like to have some sort of large scale battle. With new consoles/ engines, hopefully we'll be able to participate in some awesome Helm's Deep-esque battle sequences.
But to answer the question, maybe something similar to Mehrune's Dagon appearance in his true form in Oblivion. Fighting some sort of massive being while it is crashing through a city sounds pretty awesome.
The Kamal (Snow Demon type things that come out once a year in the summer to attack everyone), from the continent of Akavir, realize that they cannot defeat their mortal enemies, the Tang-Mo, so after using up their Summers for the past 100 of years they have stopped fighting in the summers and spent their time preparing to attack a land they heard of called Tamriel... im fascinated by the Kamal and it's a new thing we have never seen, maybe they attack High Rock? It's on the border, somewhat cold for being in the North, pretty perfect place for