I suspect that Dorian's sexuality will be highlighted in some aspect because it seems to be at the heart of some conflict he's having with the expectations for mages in Tevinter. Given that Gaider wrote him and has been wanting to write a gay character for a while, I suspect that this is going to be a really well-developed and interesting plot point.
I'm not really convinced that Sera's sexuality is going to be such an important part of her character. Her story will likely be around the plight of the city elves and, while there might some talk about her failing to create more elves, I just don't see it being as big a deal for her.
I think that Bull's sexuality will be played more like a joke. Like the Isabela/Zevran "horny bisexual" nonsense, but this time with a big giant qunari guy, since people wouldn't "expect it' from him. I think he's going to be a lothario of some kind.
I'm not really sure how it will play out for Josephine. Anyone got any ideas? There's the idea that she might be a past lover of Leliana, but I'm not convinced on that yet. She might not any emphasis on her sexuality at all.