Well if it's all a fantasy then the answer to your punishment ideas is still 'let him decide' and if he wanted ideas he could ask himself... You could talk to him about what your fantasies of punishment are and let him come up with a scenario... When people started mentioning that they felt it was very unsafe to be locking you in a basement all night without supervision, did you not realize we thought it was actually happening to you in real life?
Some of us realize it's a fantasy and say so right off and some of us are more cautious because there's always that worry that this is actually happening irl. It was mentioned in another similarly fantasy-esque thread that someone should look up the ip address of the poster in question and notify the police that unsafe and illegal activities might be taking place.
We want to help people that actually need help when they come on here to look for it.
This isn't an rp site, people here post questions and tasks for people to actually do. Some are online and some are real life but all tasks and punishments tend to be something you actually do like "hang around the supermarket vegetable isle with a bottle of ky jelly in your hand and look perplexed" <- this is a humiliation idea too
Role playing online, in just text, and fantasizing about doing things you aren't physically doing, then asking about "what should my fantasy punishment be" without mentioning it's all a fantasy, isn't something we expect to see here so people will answer you in a way that assumes things are real and happening.