It would be handy to be able to enforce all players to be a demon through a config option or the like. No choice on first death. No Opus Daemonium. Just flat out, always a demon no matter what.

A gamerule for example ?

A gamerule would absolutely work.

I am thinking such a gamerule would specifically determine the initial state for new players. Operators can always use commands to change a specific player's status, and it is easy enough to disable the recipe for the Opus Daemonium.