You need to also remember that the technology on Geni is limited and cannot do all the things we might like it to do. So we work with the constraints as best we can.
I believe several different issues are being mixed up here, as well as lack of clarity on terminology.
- fictional is what we (try) and label someone who never existed. Harry Potter is a character in a novel. Do you want to be related? I sure don't.
- mythological (or like that) is for deities. All cultures have origin stories. At some point before written history "they came to earth" as a person, who had families we can trace as best we can.
I'm most familiar with the Greek stories so will use that.
https://en.m.wikipedia.org/wiki/Helen_of_Troy was (I think) a real person and there's a well known story she plays a big role in. Was she really the daughter of Zeus? Someone thought so, or "later on" someone thought so (Remi's point); yet she was a real person and had real children etc. Where do we make the distinction? I would think about linking her to her legendary origins within the profile for easy navigation to see that tree.
Do I want to show my family a descent from Zeus? I most assuredly do not, it is insulting to them. Do I want to believe it's possible? Of course it's possible, what do I know? Geni, however, cannot show that except, I think, by the methods described, at this time.
If you want to show a descent from a mythological being in your tree, that's why personal trees were invented. :). But in the world family tree? The accuracy and integrity of the Zeus family is the concern: did we get the genealogy right? Not my own selfish - who cares - connection to him.
Sorry for rant, but I believe we are all connected, we just don't know how yet. It doesn't matter to me whether I have a path to Zeus, if I believe he's an ancestor, then he is. But we "do not" know how we connect to Zeus, we only (might) know how we connect to his human children.
There's a working genealogist I respect whose DNA test is matching mine at perhaps the 4th cousin level, I would love to piggy back from his detailed genealogy work. But he warns that "we will probably never know" how we connect.
In it's own strange way, this is a similar problem, and we do not have the software for speculation in Geni at this time. So there you go.