Bruce and Selina getting engaged. I despise the idea that all superheroes must seek some kind of stagnant status quo in regards to their personal lives, especially since the era of comics that got me hooked featured several DC characters with some kind of matrimonial attachements, and neither those books or Spider-Man at Marvel ever seems to really back up the publisher's fanatical belief that these successful, sometimes-decade-long-marriages were harming the characters or their marketability. I mean, if you wanted to find the root cause behind the Superman books' slow dive after Our Worlds at War, to me the biggest culprit is simply bad writing and far too much willingness to rewrite history without putting up a Geoff Johns-caliber explanation for it.
And I especially don't find the "loved ones will get hurt" argument to be very strong. Supporting cast members are already being targeted by villain without a ring on their finger. And again, we saw successful if dramatic marriages previously. Now, if the argue,to is that the high stress of a superhero life might doom a relationship, I could get behind that, but that's why I like the Batman test.
At least with Batman, there's enough strength in the brand and the stable of writers in that franchise to give you maximum flexibility. If you want to "have your cake and eat it too," just get the characters married, make the relationship have enough rocky elements qualify as high drama, then break them up on the page. Batman is enough of a jerk for someone to write him as an inadequate husband thanks to his workaholic nature, and Catwoman's while appeal is in being a sometimes morally ambiguous free spirit.