Age: So as you progress through the game and its story you should have a set age for example when you start in the game your 13-15. As you progress through the games you would obviously age and as you age you get stronger not by like big buffs but small buffs like lifting and striking strength. If you were to reach an age like 35 or you reach max level at age like 26 you can retire and settle down to have a family.
Family/Prestige: If you had settled down to have a family your second slot has a chance of being your previous slots son, daughter, or grandchild which then you may get special buffs for things like Your previous breathing or stats you exceled in. Doing this should give access to some special content such as Training methods, Your parents being able to teach you, special boss fights. The boss fights you could gain access to would change depending on the route you take between demon or slayer
Boss fights: the fights available if you were a slayer could be a special training fight with your previous slot who's retired. If you had became a demon after slaying your own family you would eventually have to battle your previous slot who's now an aged retired slayer, in this battle if you win you'd gain a large boost in power but if you lose you'd be wiped, and unable to be reborn in the same family line as your first slot
Extra: If you were to retire and settle down, but instead of switching slots you could become a demon and live longer obviously but doing this you'd be able to unlock the ability to use breathings while being a demon but this would replace your bda for majority of the time your a demon. There would be a way to unlock a bda by doing a special mission only after you would have lived for about 100 years in game while also meeting a few requirements. the main part of the mission would be to kill a high level slayer boss but that boss would be your descendent that would use your breathing and have insane stats beating it unlocks your bda.
just.something. You’ve Reached Level 9
