But anyway, this speaks more against the group than the characters themselves.
The likes of Kid, Law and Bonney seem to be very essential to the story now, what with one of them taking one of the Warlord spots + having the fruit that allows him to operate the Celestial Dragon's secret weapon (whatever that is) and another being extremely important to the WG to the point of being used for ransom (and this was shortly after her introduction). That's too much role for characters that were designed in less than 3 hours, and that's only 2 of them.
Here's the link to the translation of the supernovae interview http://www.apforums.net/showthread.php?t=29195&page=39
Don't just read it but think about the possibility that Oda possibly had them in mind already. Cause I said this before.. 9 rivals.. in 3 hours. Think about that. Who the hell makes 9 rivals.
What is a last-minute character?
Because if he was indeed planned for a long time, I wouldn't consider him last-minute, just his role. Same for the Supernova, as I don't believe Oda came up with all of them in under 3 hours without some prior planning. Their role as a group, however, would be last-minute.
You're free to speculate and fancy whatever you want, but unless you find a statement about how Oda had seriously planned for them, or how he would for sure introduce them later, then all we know for a fact is:
In Sabaody, Oda didn't intend to introduce supernovas at first, but only 3 hours after the editor advised him to make the story more exciting, Oda wrote a new story in which supernovas appeared. ONE PIECE Grand Countdown 2 (2010)
Sure, you can interpret this as Oda having thought them up beforehand, and then on a whim deciding to introduce them at that point…but its equally possible that he just came up with them on the spot - and then later brainstormed their different important facets. Did Kidd have an anti-Shanks alliance cooking at Sabaody? Did Bonney have specific ties to the WG? Was Law so intricately intervowen with Doflamingo? Maybe Oda just threw together a bunch of cool character designs, which he could build upon later. Introduce more tools to his writing toolbox.
Going through Odas interviews he's not the inhuman masterplan writer many take him as; you will see statements that he gets bored easily, and thinks stuff up on the fly, even that he loves drawing cool stuff, introducing cool characters, more than the story - to the point where, in his own words, everything ends up going on longer and longer when he introduces new things he fancies. Oda made Vivi the princess of Alabasta on a whim, was undecided whether or not Luffys mom was alive when questioned, and has declared that he only knows the end of OP, and nothing else.
Oda is winging a lot of it- but he has managed to wing it very, very well.