I liked Kelly's romance in ME2. She has consistent relationship build and flirting, my second favorite romance in the game.
Despite my jesting, I ultimately agree.
It's a compliment when I say it's the most "straightforward" out of the bunch in ME2, hinged on, like you said, a consistent relationship build.
She tells you how she first met the Illusive Man, some insight on her background, what she did before Cerberus and how she joined Cerberus. There was a bit more romance content (and conflicts) as well. She could also rejoin the Normandy SR-2.
Most of that is a shame, but I understand the direction they went with her, though.
Don't know how she'd fit in if she rejoined the Normandy. Assist Chakwas/Michel on the psych side of things, maybe.