Well, I like to think that I learned from past mistakes. So I'll go in with as little expectations for the ending as possible. I had a lot of ideas on how I wanted the trilogy to end and that ... well, it didn't end well, haha. So no preconceptions this time, que sera, sera.
There are two points I'd really like to see but they are more general in nature and are not really related to any specific story:
1) Ideally, I'd like for any developer, not only BW, to write at least a rough outline of the ending first, maybe even before thinking of the beginning of the story. I find it helps immensely to know from the beginning where the story should go and it helps to create a cohesive and coherent plot. It would even be great if they would create the ending fairly early and really polish it's development in terms of level design and art. I find that very often, game endings are rushed and slapped on at the very end of the development cycle, which is why so often they are disconnected and anticlimactic. This often retroactively sours my impression of the entire game. IMO, the ending of a game has to be it's shining moment. I get that there are statistics that show that a lot of players never get that far but I guess I expect of developers who want to call themselves artists that they have enough respect for their own product to make that effort regardless. Again, this is not specific to BW or Mass Effect but a general expectation - or maybe rather an idealistic wish - of mine.
2) If there are multiple endings, they should organically flow from the entire plot and not be separated by one specific final choice. Mass Effect 3 was not very good at this and another really bad example is the Deus Ex series (especially human revolution). Both series have you make countless decisions throughout the game but the endings are purely defined by one very specific 3-4 way choice at the end (in DX:HR, it's even friggin' buttons). This inevitably poses the question what all that previous stuff was actually all about (granted, ME was much more in depth with it's plot decisions throughout the game). One might look at The Witcher 3 here for inspiration. Here was one of the rare games that didn't tie the ending to one particular decisions but took a couple of carefully chosen variables from throughout the entire game and used those to determine the outcome. While more complicated to script properly, this makes an ending feel much more naturally emerging from the entirety of the story and really gives the player the illusion of choice and consequence because - just like in real life - many factors can impact on the defining final event that then can serve as a cathartic moment that nicely wraps the entire plot into one coherent picture. Books and movies do it all the time and I think the interactive nature of games offers a lot of potential here that so far is only rarely tapped.