One major obstacle is the difference in their natures. Angels are associated with good and demons with bad. Their respective communities might not accept their relationship. For example, the angel's fellow angels may think she's being corrupted by the demon. And the demon's peers might see him as going soft. Another obstacle could be the rules of their realms. There might be strict laws against such cross - entity relationships in the Gacha Life world.
Well, in a Gacha Life angel and demon love story, their relationship might start with curiosity. The angel, being pure and good, could be intrigued by the demon's mysterious and rebellious nature. The demon, on the other hand, might be attracted to the angel's beauty and kindness. At first, they might keep their feelings hidden due to the differences in their natures and the rules of their worlds. But as they interact more, they start to understand each other better, and slowly, love begins to bloom.
If Tanjiro becomes a demon in fanfiction, he might lose his sense of humanity. His relationships with his friends and family would be severely strained. For example, his sister Nezuko, who is also a demon, might be affected in unexpected ways. He could struggle with his newfound bloodlust and the need to consume humans, which goes against his original kind - hearted nature.