Kid and Law will join Loki and become his wings in the final war

Kid and Law will help in the fight against Imu and become the wings of Loki.

Loki is set up to be the true rival of Luffy. In order to be on par with Luffy he needs a crew or at least some subordinates.

The only way Kid and Law can still be relevant in the finale is if they join a strong faction. But there is not really anyone left strong enough for them to follow except for Loki.

They also both lost their ships and crew right before Loki was introduced and both are somewhat close to Elbaph.

If they were planned as wings of Loki all along it finally makes sense that Oda never gave a shit about their crews except for Bepo and Killer, who will likely both also join Loki.

Kid and Law fit much better as rivals to Zoro and Sanji than as rivals to Luffy. Kid is kind of dumb and rash and dominating and could clash with Zoro in an interesting way. Also his magnetism is a nice counter against Zoro and his swords. Law is smart and tactical and fits quite well with the cool side of sanji. His powers also work great against sanji: he can teleport against his speed and his fruit negates Sanjis germa durability.

Kid and Law both hate the world government, just like Loki and they could all want to destroy it together. Also Kid and Loki both do not like Shanks, that might help them to bond. Also Kid and Law defeated Big Mom, who Loki probably hates because of what she did to Elbaph.

Also Kid and Law would kind of dislike each other in a way similar to Zoro and Sanji and fight over who is the real number two.