One common theme is the power play. Loki is a powerful god and Tony Stark is a genius billionaire. When Loki makes Tony his pet, it's a shift in their normal power dynamics. Another theme is the redemption arc. Sometimes Loki uses this situation to redeem himself in some way, or Tony finds a new purpose while in this strange situation.
A prevalent theme is also the exploration of their personalities. Loki's mischievous nature and Tony's quick - witted and often arrogant self are put to the test. They learn from each other in the process. For example, Loki might learn about human emotions more deeply through Tony, and Tony might gain a new perspective on magic and the supernatural world that Loki represents.
The theme of friendship or a strange form of companionship also exists. Despite the odd premise of one being the 'pet' of the other, they end up forming a bond. This bond might start as a form of captor - captive relationship but gradually evolves into something more meaningful, like a friendship that defies the initial circumstances of their relationship.