I thought they did some of that in Wings of the Goddess (FFXI). At least I remember them selling the idea before the expansion.
It's the kind of feature that sounds neat as a selling point, but doesn't generally result in the game actually being more fun. Classic case of realism=! fun.
They did, with the pixies.. if people refrained from slaughtering them in the past, they'd show up in the present.
That wasn't a tough sell, pixies are super helpful to have around and there was no particular need to kill them so it wasn't really a decision.
It was an interesting idea in theory, they just didn't do much with it. Actually, I feel like they squandered the **** out of the whole time travel thing in Wings of the Goddess. That could've led to some interesting storytelling that never happened.