First thing I would like to see, port Morrowind onto the 360. There hasnt been an xbox original moved in a while and thats one of two games (the other being gladius) I wish they would port. Morrowind is my favorite of the series and I still play it on my laptop but PC gaming has always made my hands cramp.
I think it would be cool for the next installment to be a full on race war. The peace treaty is broken and everyone is tired of the others ways, could be like WoW without the mmo part. You start off in which ever race you pick's home land and work your way up the ranks of the army of that nation. Then, once you have made it to a certain status, the order comes down from the king to all imperials to try to take over where ever they are stationed. After stopping the attempted invasion, or if you picked Imperial you barely manage to survive, the remaining Imperials flee back to there home land to regroup the army. Could even add a multiplayer element like crome hounds had where you are trying to take over parts of the map, but its not like the game needs multiplayer.
It could turn out that all the gods have gotten together to see who's people are the best and put things in motion for all out war, the winning god being proclaimed King/Queen of the gods. You are the chosen one of your people and your god speaks directly to you, with the ending being your rise to the top of your people and being proclaimed king/queen. To do that, you would need to rally the best people in the trade they have chosen and the only way they would follow you is if you are the head of the guild they are a part of. All the NPC's would have to choose, loyalty to there country and people or Loyalty to there guild.
You would jump from land to land taking over and enlisting the help of the people there after you take over and gather them all to eventually take on the Imperials, who have the biggest army of all races.
I may have been to specific in what Id hope to see, but all that was made up off the top of my head in about 15 minutes. Im just a construction worker so Im sure they can come up with something way better than what I just did. Either way, I'll be happy just to have whatever they come up with.