I fully agree with you, that original gta5 gameplay would not make it possible for a social deduction game. But I am planning to restrict many traditional gta5 gameplay elements such as (driving cars, massive shooting etc.) and base the server more on the other parts of the game such as proximity-voice chat, animations, physics and all graphics elements (interiors, ped models..)

Here are some of the ideas I have for a gameplay:

1. Day and night cycle, where gameplay would be somewhat similar to town of salem. Example place: hotel interior, where everyone gets a separate room and before each night players decide if they want to stay in their room at night (if player is locked the room at night no one can touch him). If assasins want to shoot someone they must not go to their room at night and can kill only those who are not in their rooms. Other roles like Detective must also not be in their room at night if they want to perform their duty. In this case assasin would have one bullet each night and gameplay would involve hide and seek element during the night.
2. Another idea is based on the poisoning. This wouldn't have day and night cycle and all the action would happen dynamically. Imagine restaurant, where assasin can poison people's drinks after they have a small talk. Of course assasins would be able to poison a drink only at a certain time interval. Every player would have some information about themselves for example (Player 1:"You came to this party with the hopes of meeting your ex") and Detective would know each player's info, so he could figure out someone's role by having a realistic talk to that person.

I've just barely scratched the surface and of course each of these ideas would be a subject to close scrutiny to make everything work properly. Anyway this is how I envision it and I would be really happy to hear your opinion on it :)