Sorry if it's a morbid question but I'm at that age when death is coming closer each day, and I'm wondering what to do with my fics on AO3 and tumblr should I die one day. Of course I can put them in a will or get someone's help. But me writing fanfics is

Not morbid at all! Planning for the future isn’t depressing, it’s planning. Taking care of something that’s on your mind will only make you feel better about things, anyway :)

As for the answer to your question, AO3 has exactly what you’re looking for: Fannish Next of Kin. This is the person that you decide you want to have take care of your works after you die. They’re a fandom friend you trust, but you don’t have to know them in person. You don’t even need to know their real name. You just have to trust that they’ll follow your wishes after you’re gone.  

In the event of your death, the Archive will transfer your account into that person’s control and they can manage it on your behalf. 

For more information, read that link above. Just one more way that AO3 looks out for the “Our Own” part of the Archive :D.