Being an old married dude, it's been a long time since I've had call to use a "pickup line", but from what I remember of the process, crafting would be completely orthogonal* to what actually matters: whether or not the person you have just attempted to initiate conversation with finds it attractive that you find them attractive.
"Gotta light?" would almost be perfect, because it allows your potential impressee to choose from a wide range of responses, from the brush-off through the guarded interest to the hello sailor, with absolutely no pressure or social repercussions.
Unfortunately, that line worked better in a world where people smoked cancer sticks. On the other hand, I've seen two macho friends (who flirt just because they enjoy seeing if they still can, not because they're trying to get anywhere) work wonders with "Good evening, ladies". (note the parallels to "gl?")
* an english-language chengyu: "fish or cut bait". Although I certainly applaud the hacker's spirit that went into building this, IMO spending effort on lines is not fishing; it's needlessly cutting bait.