Which means Rocks was a plan EVER SINCE that time
Because of two reasons:
1- Oda already planned for Oden... thus the 2nd division commander was vacant for too long before Ace filled the position and ONLY now this story line is paying off
2- Young Whitebeard in his flashback before he died... he was talking to a crew calling him "Newgate" instead of Pops or Oyaji... and BEFORE Rocks even came to story... I remember creating a thread in OJ where I said Whitebeard sailed and was NOT a captain at first.... and almost everyone wanted to Kill me for it... the thread was closed LOL... ANYWAY, I'm not here to rant... but EVER SINCE Marineford.... Oda layed out some set up for CURRENT events of Rocks pirates by having WB working under someone.... and only now, the story of 2nd division of Ace is being payed off... so hell yeah Oda planned it since that time
I'm not the sort of guy who says "Oda planned this number of chapter if you flip it, it will look like a snake, so it's Boa Hancock" type of fans LOL... but the story of Ace/WB was being set up since that time.... Hell, EVEN Oda hinted for Ace being involved in Wano and to have a flashback about him since he learned to make hats for Oars... which is most likely by Otama by the time he spent with her...
So some cool story lines were laid out and set up since marineford arc... and I do believe Oda planned them
Eh. Just because there was a vacant seat prior to Ace doesnt mean Oden was planned, and by extension that doesn't mean Rocks was planned either.
Ace was given a a vacant division seat, thats the only motivation needed to really to create the vacancy in the first place. He could have retroactively used Izo and that seat to make a convincing story about Oden. Again, not saying he didn't plan ahead, he did plan Rayleigh that far ahead it seemed (though you could argue the same thing, that he simply used an old character design for Rayleigh).
For example, the impel down escapees could just be "blank" pirates Oda decided to release and forshadow, but hasn't tied them to anything. He could just retroactively make them all Rocks pirates just to even further solidify that he "planned" Rocks or is at least using his characters or plot points wisely