The best possible possible game with a cyberpunk setting?
Definitely huge cityscapes...maybe one big, open-ended world like an Elder Scrolls game.
Lots of customization in terms of cybernetic enhancements/prosthetics/skills/weapons and gear and even appearance would be nice(male/female and facial/physical features, rather than just future geralt every playthrough).
Transhumanist stigma and corporate wars like any good cyberpunk setting should have. Maybe have a group of eugenics trying to compete with the cybernetics with total naturals harassing everyone everywhere.
Multitudes of factions and side-quests to play around with like cops/government, gangs, special forces, PMCs, various corporations to join each with certain benefits.
Choices that mean something to that playthrough, maybe choices that'll come back to haunt you in the future(wish I'd have killed/saved that guy when i had the chance! that type of thing).
Many different ways to approach any given mission/quest(sneaking vs run-and-gun, lethal vs non-lethal, hacking computers vs smashing doors open, etc..).
Basically, as much customization as you can put into it will make people go nuts for the game. Appearance, mission strategies, equipment, skills and everything in-between...the more choices and customization available, the better.
There aren't many cyberpunk themed games so people definitely want something they can sink their teeth into and play over and over again while still keeping a somewhat fresh feel each time.