Dean isn't going to fight Sam.
What will happen is Sam cures Dean of demonism and finishes the trial, thus dying.
All demons get locked in hell, Castiel keeps Heaven in check and Dean remains on Earth to live out his days, until he dies and joins Sam in heaven.
I feel 1 season isn't enough to deal with:
1. Michael and Lucifer, who are still stuck.
2. '' 2014 '' - '' You will always end up here ''
3. Gabriel possibly being alive.
4. Demon Dean, Angel Residue in Sam.
5. Crowley.
I could see that being the finale, but first Sam would have to find a way to contain one of the most powerful demons on earth.
1. Michael and Lucifer are going to stay stuck, I doubt they'll be addressed.
2. I don't actually know what you're talking about.
3. They MIGHT address that hint, but they don't need multiple seasons for it.
4. Wasn't Sam rid of all Angel residue? Regardless, there's no reason Demon Dean needs more than a season of carnage to be done well.
5. I'm pretty sure Crowley's growing humanity, not to mention his son's presence, can be addressed alongside Dean's waning humanity. No reason that needs to be a separate season.
Without pointless Angel War episodes taking up space, a there's more than enough room in a season to address these issues.