Ugh. Do not watch Grave of the Fireflies. It is so dumb. I only say that because I cried like a small child and its just too sad. Don't do it. Also, you haven't seen KiKi's Delivery Service?! Omg! 

And yeah I've read a few of those fan theories. I've always wished Ghibli would do sequels. Just for that reason. Totoro is actually supposed to be a calming spirit of something for children that have seen or are coping with a death. That's stated pretty clear in the movie. I won't ruin it for you though!