Content, not makework like having to relieve oneself. Hybrid Heaven wasn't very long but did let you level up individual limbs as suggested.
For the 'deepest RPG imaginable' though I'd prefer length. Have a complex skill system. After about the first third of an extremely long, multibranching story start introducing skills that make your current ones gradually become obsolete. After the second third, do it again with a third tier of even more powerful skills and equipment. Story-wise, have a time skip between each segment and multiple side quests and bonus dungeons in each one.
So basically three long RPGs stuck together with connecting characters and systems. Kind of like the Baldur's Gate Trilogy except all one game. In that one you kept your character all the way through, and by the end of the final part he was over Level 47+ and fighting advanced enemies that were still challenging to him. Consider that most D&D character sheets go up to 20.
For the 'deepest RPG imaginable' though I'd prefer length. Have a complex skill system. After about the first third of an extremely long, multibranching story start introducing skills that make your current ones gradually become obsolete. After the second third, do it again with a third tier of even more powerful skills and equipment. Story-wise, have a time skip between each segment and multiple side quests and bonus dungeons in each one.
So basically three long RPGs stuck together with connecting characters and systems. Kind of like the Baldur's Gate Trilogy except all one game. In that one you kept your character all the way through, and by the end of the final part he was over Level 47+ and fighting advanced enemies that were still challenging to him. Consider that most D&D character sheets go up to 20.