I've wanted Cullen as a companion/romance option since he was first introduced in Dragon Age: Origins. If you play as a female mage, Cullen has definite feelings for you and makes them known during both times that you come into contact with him during the game. However, nothing ever comes from it (because it was forbidden). Besides, he was more of a background character and was not all that important to the plot.
He becomes more of a prominent character in Dragon Age 2, but once again, he was not able to be recruited as a companion and thereby remained ever un-romanceable. However, his character was given the opportunity to grow and you saw him as someone who was more conflicted than as the rigid and unforgiving Templar from the first game. In DA2, he genuinely wants to do the right thing, but he's not willing to go to the extremes that his commanding officer is willing to go to; rather, he would prefer to find a more reasonable, middle-ground.
Cullen has great potential to be an awesome addition to your party, and if he becomes a companion in Dragon Age 3, then that will be 1 more reason for me to be salivating over the game and its release.
Please, please PLEASE make Cullen a companion/romance option in Dragon Age 3?