Luffy in the manga is strong, for sure. He's constantly evolving and unlocking new abilities. When compared to others, it depends on the context and the specific situation. Sometimes he might have an edge, while in other cases, his opponents could pose a significant challenge.
Well, in such fanfiction, Luffy's character often becomes more aware of his emotions. Usually, he is so focused on adventure and being the Pirate King, but when he falls in love, he starts to think about someone else's feelings. For example, he might be more hesitant to go on extremely dangerous adventures without considering how his loved one would feel.
Their relationship could develop through mutual respect. Luffy respects Whitebeard as a powerful and respected pirate, and Whitebeard respects Luffy's determination and potential. For example, they could start with a simple exchange of stories about their adventures at sea.
Luffy and Wanda's interaction could also be centered around their different powers. Luffy's stretchy abilities are very physical, while Wanda's magic is more mystical. They could learn from each other. Maybe Luffy wants to learn some magic tricks from Wanda, and Wanda is interested in how Luffy's body can do such amazing things. This exchange of knowledge could lead to a great friendship in the fanfiction.
In many Luffy fanfictions, a jealous Law can be quite an interesting character. He might be jealous of Luffy's popularity among the crew or his carefree attitude. This jealousy could lead to some tense moments between them, like when Law tries to outdo Luffy in certain tasks just to prove himself better.
Luffy beats Kaido by unlocking new powers and exploiting Kaido's weaknesses. He also gains support from his allies and uses teamwork to his advantage. The battle is intense and filled with unexpected twists.