Sometimes I get a hook from browsing the rules and seeing if I can max out a niche. Then I tone things back to give enough resources to the character in other areas and work out a good story as to why they're like that: Redline, the fastest mundane; Pockets, who has that bit of gear.
Sometimes it is the style: Dino-butt (street name given by fellow players), the so-so rigger/decker.
Sometimes, I look at what the GM or official play rules say can't be done with specific character options and either try to accomplish a character with a similar look (to others) or feel (to me); or come up with someone with the skills and resources to be a counter to such: an anti-Drake project, where a research group had tried to either duplicate or counter Drake abilities with cyber/bio-ware.
Last I can think of is how something in the Sixth World might affect a person causing them to not function well in corp-drone society: I've had an idea for a while of a somewhat cracked shaman who has, or thinks she has, Dragon as a mentor spirit and is gradually adding 'ware and making cosmetic changes to be more like her mentor. Other Awakened people could have less drastic but also isolating responses to what they can perceive: acting in line to what their mentor spirit wants/acts; avoiding nastiness that mundanes are blissfully unaware of (such as a possible background count in the corporately cheerful Big Box Store).
A character's background, abilities and flaws also tell the group and GM what part of the Sixth World you want to explore. Do you want to check out High Society by posing (or being, while the pay holds up) one of the Upper Crust? Do you want to see how the ignored masses deal with being one of those statistics swept under the rug? A suburbanite who discovers how fragile the plaster shell of their lifestyle is? There's environments, too. Sprawl experts are a dime a dozen, and how's the dollar to the nuyen these days. Woodsman, perhaps; an expert hunter can turn their skills to more humane pursuits, if a pun may be allowed. Water, arctic, desert, tropical are other environment specializations.
I hope these ramblings help!