Both of their fears kept Tony quietly contemplative. Billy's showed his immaturity again-- not the fear itself, that was entirely valid. Tony knew too well what it felt like to lose control. If the muddled story of Scott's death in their world was to be believed, Billy already knew what it looked like when someone with his power lost their grip. But he fought so adamantly against what that world's Tony believed in, and what turned out to be their reality: registering powered individuals to make sure something like that didn't happen again. This Tony wasn't convinced, but even Cassie had accepted the training after accepting that she had lost the fight, so why hadn't Billy when he was afraid of himself? He couldn't even connect the two concepts. He probably couldn't see five seconds ahead of himself.
Nick's was similar in theme, and familiarity, but Tony was sure that Nick didn't really have anything to be afraid of. Maybe it was the fear itself that made sure of it. That was probably what made him who he was, so precisely in control and aware of every possible angle and failure to be there with checks and stops so that even he could be neutralized should the circumstances arise. He was born that man.
Tony wasn't. He said, quickly, skipping through it as he had before to move on to a new idea, "Wedding bells. Maybe just the word 'Congratulations' with an exclamation point and gold foil. What's your biggest regret?" When he finished, it was less like a push to move on than a bolstered rush, like ripping off a band-aid. He stared at Nick as he asked, letting him set the tone for this round of answers before Tony did, and wondering if there really was something in his life that had defined his fear.