Kid to Luffy is more or less like WB to Roger while BB to Luffy is more or less like Rocks to Roger.
Can we really say that WB and Roger are not rival just because Roger has rivalry with Rocks before and WB didn't really share similar goal with Roger? Obviously we couldn't. They are still rival in certain way.
While not exactly similar, there are some parallel between Luffy-BB and Luffy-Kid relationship with Roger-Rocks and Roger-WB relationship.
BB will be the first and one of the toughest anti-thesis from Luffy while Kid will be more a "healthy" rival for Luffy.
After Luffy-BB business settled in near EoS, Kid then will be known by the person who will come after this era as Luffy's rival (more or less like WB was known as Roger's rival by people who live in current era, after Roger's business with Rocks was settled).
Also, if my mind serve me right, Supernova was planned by Oda. He just didn't planned to reveal them in Shabaody, but later in the story. Cmiiw.