So I'm starting up a game, and I've been helping my players (who are all new) with character creation. One of my players, who is planning to run a street sam, was doing his shopping and saw the blurb in the book where it mentions that GMs and players can work out custom items and equipment at the GM's discretion. My street sam player joked that he'd love to give his guy a retractable chainsaw arm, like the character Jack from MadWorld and Anarchy Reigns. Now, this doesn't seem entirely unreasonable to me (I know CyberPunk has a chainsaw arm option), but I'm wondering how I would go about ruling on something like that without either making his character completely broken or making the chainsaw rather useless (since the chainsaw as a weapon sucks as it's written in the rulebook).
Any other GMs ever try implementing this or something similar? How did you handle it, and what would you suggest?