For starters, I don't consider them fictional. Perhaps more precisely, I don't categorize religions into boxes labeled "fictional" and "non-fictional." Religions are religions. Any and all of them can be followed or drawn from for inspiration as one wishes and it is all more or less real.
That said, the foundational assumptions and grounding of a religion are important to understand when incorporating it into one's practice. All human religions more or less emanate from the human experience (because they can't do otherwise, really). The original religions of humanity stemmed from raw experiences of reality, both this world and the myriad otherworlds. In the case of so-called "fictional" religions, they are grounded more firmly in the otherworlds by their nature. Whether or not this is a problematic issue depends on what the person needs to make meaning in their lives. Generally, a religion too divorced from the person's life ways is a bad fit. But a theological structure based on Elder Scrolls makes a tremendous amount of sense for someone who is religiously devoted to the study of and participation in that otherworld - it is part of their life way.
I recall some years ago being part of various conversations about pop culture magick and pop culture religion. There is, sadly, a fair amount of bias against such practices even within communities that one might think would be more open to that sort of thing. Not sure if that needle has shifted at all, but I've always been 100% behind it. If for no other reason than the polytheistic theologies of antiquity were the pop culture of their day. Why exclude something based on modernity? Ah yes, there is the Pagan trope of ancestral authority I suppose... (and other paths are prone to the "it's old therefore it's legitimate" trap too). But there's on reason to limit oneself in that way unless one wants to. New, old, ancient, modern, this-worldly, otherworldly... explore and have fun!