Personally I'd like the leading man to be a bisexual Atton or Bishop, a medium to long haired bad-boy whose got the good looks, not afraid to get his hands dirty, and definitely not be whiny or have trust issues. In other words, I don't want the typical Bioware male romance (Carth, Kaiden, Jacob) in which they are overly generic and while likeable (except for Carth, don't know how anyone liked him) they don't particularly stand out either.
Definitely if he's a warrior, more like a two-hander or a dual wielder (rather than the protective tank) and if he's a rogue, he's a good fighting rogue. He'd also make a great arcane warrior (complete with pitch black steel armor, a huge sword and some deadly blood magic). Come to think of it that would be perfect, an arcane warrior.
The catch to this guy is that Hawke CAN potentially turn him good or neutral depending on her (or his) choices and influence him. Perhaps he sees the light and becomes a paladin, retaining that gruffy edge but using it for redemption, or becoming neutral in which he no longer goes after innocents but still keeps the bad boy life. And being romanceable to both genders (like Sky in Jade Empire) just gives him much more versatility, except unlike Zevran he is not openly bisexual, or may even be straight but just attracted physically and romantically to male Hawke if you persuade him well enough (or show disinterest to female party members).
I would also like to see a darker featured male romance, potentially a Rivani or even someone not native to Thedas with skin ranging from deep tan to very dark, dark hair and piercing hazel or brown eyes, the epitome of "tall, dark and handsome", complete with a deep, sultry voice that makes you just want to die in his arms. I'm thinking a bardic type for this guy who knows his way with words...and song. And because he's very attractive you have to compete with Bethany and other female party members, potentially a sidequest in which his lost love comes searching for him and you have to fend her off.