I think one way to step up the realism of shenmue is to create an eating system.
Ryo would have to eat three meals a day. There could be two metrics - nutrition, and enjoyment. On days that are very physically active (a day with a fight, or a work day) it would help to eat more nutritiously. However, if Ryo eats the same thing all the time, the enjoyment metric goes down.
It could have an effect on everything. Eating nutritiously could give you a health bonus that day, while NOT eating at all gives you a penalty. If you have eaten properly, you could run continuously, but if you eat poorly, you have to stop running from time to time, like in GTA.
I like this idea because it adds a layer of reality to the game. It also creates a system whereby the player will need to manage their money more effectively. You'll want to buy the healthiest, cheapest thing most of the time. But if his enjoyment of food is low, you might have to bump up to the more expensive dumplings.
Most importantly to me, Shenmue was a series that recreated real life better than any game before, and also mostly since. I think that because no characters ever eat (going to the bathroom could be shenmue 4) in any video games, this would be a very simple way to increase the immersion, and once again be ground breaking.
I would want the mechanic to be as simple as possible so that it doesn't command the attention of the player ALL the time... but it would be nice to worry about once or twice during an in-game day.
I'm not entirely sure how it would work, but it might be fun to think through some ideas with you guys. How would drinking soda fit in, for example? I could also see Ryo being introduced to many different kinds of foods now that he's in China, so far from home, that would add depth to the world, and create in the player a sense of travel.
Another thought is that different kinds of food could provide different buffs. This kind of dumpling makes Ryo run 10% faster, but it lasts for six hours. This kind of food makes Ryo's kick moves do more damage, lasting for 2 hours. It would be a fun way to modify how the game works, much like a JRPG.
Let me know what you think.