Here are just some things I've thought about the White Knight trope and particularly in relation to BioWare.
White Knight characters tend to be connected to the overarching plot of the story, and thus be "forced" companions or ones you can't get rid of until the end. They also, disproportionately have their personal life involved in the main plot (Alistair and the Landsmeet, Carth and his thing with that Admiral guy who killed his family, Aveline and Donnic, and others I'm probably forgetting). It makes sense like this because the archetypal "white knight" is that actual knight who rescues a Princess and marries her to become King. So their personal "quest" is involved in the overall "plot" of the future of the kingdom. Often times their personal quest IS the overarching plot. In BioWare games, they tend to be the character who the story might focus on if the story did not have to accommodate the Player Character. DA:O is, unfortunately, the best example BioWare has done of this trope, because of Alistair, the way his personal development is handled is undistinguishable from the plot at times (the Landsmeet, Redcliffe), and the way he's become a "face" of the story (showing up in, potentially, all three games, and having his own comic series, where he usurps both player characters by palling around with both the Warden and Hawke's companions). Unfortunately, I can't see any company, not even BioWare, making a s/s white knight on the same level as Alistair.
But I support AT LEAST a lessor white knight s/s character (like possibly Cullen, possibly Cassandra (if they do not end up being full-fledged white knights)) and would be oh-so-giddy if a s/s romance was ever treated on the same level of plot-centrality as some of the straight romances have been in the series (Alistair, Aveline).