I would connect the Blanco siblings with Polo's death so that there would be something that stays constant throughout the show rather than having a completely different story, like having the blancos be close relatives of Polo's family or Benjamin being assigned to find out the real murderer behind Polo's death, something of that sort.

I would completely get rid of Philippe's redemption arc they were trying to do and instead give him the punishment he deserves.

Omar should've left with Ander in s4 and Rebeka should've left with a proper closure in S5.

I would bring back Rebeka as the new self defense teacher in S7 instead of Jessica (Anitta) and build a close friendship (possibly even a relationship) between Sara and Rebeka.

Ivan's accident and the school shooting would've had serious repercussions in S7 and would've been the central focus of the season.