Well if they deliver the "40 possible endings" then i guess it is possible to finish the game with a bad ending, as in, you fail, the world is doomed, despite your efforts, its seems they werent enough. Everybody that matters either die or run away from the Elder One. Or maybe you can actually kill the Elder One either way but at the cost of pretty much everything - companions/factions/whole countries/etc all being destroyed.
Bioware has proven they can make a very satisfactory ending that totally depends on how you play the game in Mass Effect (ofc im talking about ME2 and the suicide mission, not ME3 endings). Depending on your relationship and your efforts towards your companions you can have a very drastic ending.
The first time i played it I was so shocked with what was happening all around me (everybody just dying!!!! minutes away from each other) that i had to stop before i had a heart attack and reload another save way back into the story to make sure i was going to be able to save them all.
If they apply the same kind of Final Mission to Inquisition (but not just about the companions ofc, you got your power, your influence, resources, armies, etc) you could actually have that kind of ending as in, you didnt lead them (or not good enough at least) so you fall....
In a way, im hoping the ending of Inquisition is gonna be like a between the lines message from the devs: "You see this ending? You see the consequences of your choices through the game? You see what happens if you take the time to get this much power and reputation and armies to fight? Yes, thats what we wanted to do with ME3 ending but then.. reasons.. and it didnt happen... So here you go now, enjoy! We can deliver!"
I hope....