Oh look, someone trying to find a deeper meaning to the point of ridiculousness.
Totoro is not a shinigami. He's a spirit of the forest. In particular, that big freaking tree they found him in. So basically, he's a
kodama.
If there's any hidden meaning behind it all, it's about the importance of nature. Totoro, being a nature spirit, is shown as a friendly and helpful creature. The girls plant seeds for later trees to grow - they even assisted him in getting them to sprout. They greet the tree in which he lives with respect. The entire village is treated with a kind of nostalgic charm, as they live close to nature and away from all the bustling city.
Furthermore, the film is about the innocence of childhood. None of the adults can see any of the strange things occurring, but the two young girls can - though if I'm not mistaken, it's suggested that after the incident, they never see any of them again. They grew up, I guess. He's like an imaginary friend. You're in a place you're unfamiliar with, you don't know anybody but your sister, so what do you do with your time? You make your own friends, and the two of you go on adventures. And they're fun for a while, but eventually you grow out of them, find real friends, and move on. You may never forget your imaginary friends, but you just like what can happen with real friends, you just kinda drift away.
EDIT: It took me a bit to weed through the panicky gullible masses, but
I found someone who agrees with me.
Someone else too.
And even
Yahoo Answers takes the time out of their crackpot bullshit answers to give some rational enlightenment!