There's no way Nika was planned before Wano, probably not even before Who's Who. There were many places he could have tried to foreshadow this otherwise - Fishman Island with Fisher Tiger who's symbol is a sun, Hancock, Doflamingo fight/Katakuri fight where awakening of re-hinted as the next powerup for luffy. Even the many flashbacks about Joyboy/laugh tale, or when the gorosei are explicitly talking about luffy. Or even from Blackbeard who has an usual level of knowledge about legends.
Also yes Luffy was shown to be sort of "destined" but there was always a level of seperation from literal destiny, although the destiny beats did get strengthened over time. Luffy was chosen in the sense that no one else had the same way of seeing the world, sense of determination and incredible luck. This is another massive leap and also diminishing the value of his effort.
I'm thinking that the idea was to somehow tie awakening into luffy beating Kaido (would make sense since breadcrumbs for this was dropped since Punk Hazard and impel down) but Oda couldn't find an interesting or satisfying way to tie that to luffy's current fruit and the situation. Luffy with the ability to make things vaguely have the properties of rubber (inflatable, stretchy, bouncy) is more interesting to play with than luffy having the ability to turn things literally into rubber. Also gives him a massive powerup physically for good measure.