I think the original intended ending would have been pretty good. Since the big bad was the most relevant to Sam & Dean's character's & I believe the ending of Sam sacrificing himself & overcoming lucifer through his memories of the impala & Dean going to live a normal life due to his promise with Sam was pretty good way for the series to conclude the relationship between the brothers and all the characters involved in general.
So for the end of the series, i would want Sam & Dean to be up against a Big bad that ties to the very first Season & the only potential Big bads that I can think of that fits that bill are Lucifer, God, & Final Sam & Dean Conflict. So I see a coupe good endings possible:
1. If it turns out Sam'shallucinationsare actually the real lucifer speaking to Sam & eventually he manipulates Sam or Dean to opening the cage somehow & remerges as the big bad. But this time Sam & Dean find a way to put him down w/o sacrificing ether of them, than perhaps that would be a good ending to show. Not sure if it would be a better ending the original ending, but it would tie into everything that's happened so far nicely enough & the message of the ending instead of sacrifice for the great good like Swan Song could be avoiding any sacrifices.
2. We could have a scenario where the brothers confront God in the final episode & rather than him being a true big bad they have to kill it's more like them confronting him about how he allows so many people to be killed by the monsters he created. This could be interesting, but I feel it's the weakest of the possible decent endings.
3. Personally the ending that i'm routing for & the one I believe the writers are working towards is the final big bad being Dean & Sam. What I mean by this is that with Sam being a bit insane in the membrane from the wall coming down, fake lucifer's influence, & the divide between him & Dean forming again since Dean killed the Kitsune chick I could see Sam making the wrong decision again like he did in S4 which ends up causing people to get hurt or helpingleviathans come closer to achieving their goals. Meanwhile Dean indicated in the episode that he killed the Kistune chick that he doesn't believe someone can change their nature & once a freak always a freak, So I could see Dean after Sam makes this mistake coming to the painful decision that Sam can never change his nature & Sam is the one he has to defeat. So in the final episodes we would have a scenario where both brothers are misguided & a fight ensues over the fact of whether or not Sam can really change or if history will just keep repeating to thedetermentofthe world & Dean is just being selfish by constantly giving Sam a pass on his actions. In-fact given that there is an episode coming up titled Time After Time, I'm wondering if Sam will begin to go down a dark path yet again in that episode causing Dean to begin to wonder if things are just destined to repeat themselves time after time.
I think this would be a good way to finally bring the brother's relationship to a head & it ties into everything that's happened so far in the story line. On top of that it would giveopportunitiesto give many characters a good conclusion like Bobby & perhaps Castiel appearing to Dean to talk about not giving up on Sam. Also for once Dean's actions will be called into question of whether they are right or wrong, which has an intersting dynamic to it. On top of that I just really think this is the direction this season is headed, considering the circumstances of Castiel's Death was obviously intended to make Dean believe that people can't change & history is just destined to repeat.