As for mechanics, I like difficulty rules unpacked. So have Easy/Normie/Hard, but give say fast travel, hunger, & companion death checkboxes (for say an RPG). Also players should be able to change them on-the-fly, not just at game start. Only ever seen this with Minecraft’s /gamerule

my favourite game for this is silent hunter 3

depending on how you set the individual realism settings it can be an arcadey "sail about and blow shit up" game, or an incredibly tense full on simulation and anywhere in between