None. Every relationship is different, and so is every romance different. I wasn't expecting Thane to turn up in Mass Effect 3 at all, but he hung on, and ended his life in the most heroic way he possibly good.
It's not a relationship simulator, it's a story simulator. We've got doomed relationships, forgotten relationships, rekindled loves, secret relationships... and they all make a lot of sense for each character. You knew Jacob's history and his character before you got into a relationship with him. You knew your long term prospects with Thane were limited. And you knew how important Cerberus was to Miranda - and, equally, how important Miranda was to Cerberus. Doesn't mean their stories were predictable, but they're risks that were taken. And isn't that what every relationship is? A risk.
I'm quite pleased that they weren't all these fairy tale romances, weren't these big stories where you fight the reapers, hand in hand. It's a lot more 1984 than that - little snatches of joy in a terrible era. Even the people you get to keep on board, it's not like you're at it like rabbits. It's sweeter than that, a little understated - and for me, that's more of a strength than a weakness.
I mean, sure, a little more interaction would be great. They've written brilliant characters, who I'd always love to see more of, so it'd be nice if you could catch up with old flames throughout the game over Anderson and Hackett's holographic intercom... but I don't think it's necessary, and it's certainly not an entire DLC's worth of content. Could be a feature bundled into a different DLC, perhaps - but it's not any higher a priority than more story, more action, more planets, more exploration. I love Mass Effect - more anything's good!
That said, if there were any romance I think they could've done more with, it'd probably be Jack. Not speaking with any kind of bias here, she wasn't even alive by the time my main Shepard reached Mass Effect 3, she just feels like the one there could've been a little more story with. But I don't think we need anything more for any of them, and I don't feel like there's anything they should've done that they didn't.