In some cases, it could be a jealous Hogwarts student. This student might be envious of Harry's special bond with the dragon. They could try to undermine Harry and the dragon's relationship, maybe by spreading false rumors or trying to get the dragon in trouble with the school authorities. This student could be working with some dark forces indirectly, as their envy makes them vulnerable to being influenced by the wrong side.
In such a fanfiction, perhaps Harry discovers a dragon in the Forbidden Forest. He realizes the dragon is being hunted by dark wizards. Harry, using his skills and magic, decides to protect the dragon. They form a bond as Harry spends time understanding the dragon's needs and communicating with it through magic. Eventually, the dragon becomes his ally in fighting against the dark forces in Hogwarts.
Maybe a group of dark wizards who are jealous of the power of the dragon knights. They form an alliance to destroy the dragon knights and anyone associated with them, making Harry and his newfound friends their main targets.
It could be Voldemort still. Even with Harry having the power of a dragon god, Voldemort would be a major threat. He might be envious of Harry's new power and try to steal it for himself.
Perhaps a group of anti - dragon fanatics within the wizarding world could be the main antagonists. They see the dragon as a dangerous and uncontrollable creature and will do anything to get rid of it, even if it means going against Harry.
Maybe it's an organization that believes the dragon's blessing is an abomination to the wizarding world's natural order. They see Harry as a threat and will stop at nothing to eliminate him and the power he has received. They could be a secret group within the Ministry of Magic that has its own radical ideas about magic and purity.
There could be a group of pure - blood wizards who believe that dragons are beneath them and that Harry is tainting the wizarding world by associating with them. They would try to stop Harry from his dragon - related activities, and they might use underhanded tactics like casting spells on him or trying to get him expelled from Hogwarts.
It could be another wizard who wants to use the dragon for evil purposes. This wizard might try to stop Harry from taming the dragon so that he can have it for himself.
Another possibility is that there could be a magical creature that has some sort of grudge against Fawkes. This creature might see Harry as an obstacle because of his bond with Fawkes. It could be an ancient and powerful creature that has its own motives and goals that conflict with Harry and Fawkes' relationship.
It could be a dark wizard who wants to control the holy dragon for his own evil purposes. He might see the dragon as a weapon to take over the wizarding world.
The 'bonds of matrimony' can also be a way to explore the characters' growth. In a fanfiction setting, we might see how characters like Harry or Hermione develop in a married state. They could face new challenges together, and the bond might be tested by things like dark magic threats or political unrest in the wizarding world.