Thinking about it.
Didn't the Fallout games make more money than Oblivion?
I mean, the may not have sold as many copies, but, I imagine the development costs were much higher than Oblivion and therefore the Fallouts made more money.
Given Fallout is a very "Streamlined" version of Oblivion, simple 2-piece armours, no archive of spells, no enchanting, a simpler open world, without too many dungeons and side quests, generally speaking, the entire game is just Oblivion set in a different time and with a lot of the more RPGish features removed. (Not to say Fallout doesn't have RPG features, but, compared to Oblivion.)
One can't help but think that this would be a factor is deciding how to make the next game.
On top of this, you have to consider that the average console gamer isn't terribly familiar with the RPG genre and thus things like Enchanting, debuffs and other such stuff like that aren't things that just fly freely with them. Thus, to make everything simpler is to prove more appealing.
Not to say it's going to be a bad game. But, I really do prefer a complex and indepth RPG. Which is one of the reasons people have always looked to the Elderscrolls series.