I wont´s say anything about the food/thirst/sleep system since those things have already been discussed in many other threads.
For the poop system:
I think it would be kinda odd to watch your character taking a dump behind a bush every once in a while. As @Jenndragonfly already said : something like this is perfectly fine for the Sims games but not for an RPG.
The same goes for pooping Titans. They are supposed to be dangerously looking creatures. Seeing said creature defecating before I (& my group) engage it would kinda destroy that image.
But I have to say that I like the idea of creatures that poop valuable stuff when they eat certain things. One idea for such a system could be fuel refining (for furnaces for example). Let´s assume such a creature would eat wood and one woodblock has a fuel value of 1. If you would give a wood block to such a creature it would “process” it for a certain time and after that it would “place” a block that has a fuel value >1. This would make quite an unique way of using livestock creatures since in most games such creatures are just used for food production. For the design of such a creatue I would have something like this in mind:
I have to admit that I fell quite in love with that creature while writing this, might even create an own “creature suggestion” thread for it 