As GWS appeared to be firming as the new home for Petracca's Demons teammate Clayton Oliver, West Coast co-captain Oscar Allen joined Brisbane, and two-time premiership Lion Brandon Starcevich went to the Eagles.
The Saints gave up draft pick No.7 on Wednesday to secure Flanders, meaning Gold Coast now hold four first-round selections.
That will give the Suns more capital to secure a trade with the Demons for Petracca, who nominated Gold Coast as his preferred new club ahead of Adelaide.
Flanders joins Carlton pair Tom De Koning and Jack Silvagni as prominent recruits for the Saints.
He struggled this season to break into the Suns midfield, dominated by Brownlow Medallist Matt Rowell and fellow All-Australian Noah Anderson.
"It's very exciting. I think they're definitely heading in the right direction, so it was a big factor in choosing the Saints," Flanders told AFL Trade Radio.
"Nas (Nasiah Wanganeen-Milera) staying, like, he's a superstar, one of the best in the comp, so for him to stay, he sees something there and, yeah, can't wait to work alongside him.
"I see myself as probably more of an inside midfielder. Gold Coast have got arguably one of the best midfields in the comp."
Since Oliver was told by Melbourne and their new coach Steven King to explore his options, he has visited the Giants for meetings with key personnel and to look at their facilities.
The Magpies were said to also be suitors for Oliver, but coach Craig McRae was unsure on Wednesday about whether their list manager Justin Leppitsch had made contact.
"I'm not privy to the conversation ... it has been hard to get hold of Clayton, I will say that. He must be having a good time in Sydney," McRae said.
Oliver has made it clear he is impressed with the GWS facilities, while also taking a swipe at the Demons.
Asked on Tuesday what he liked about Collingwood, the 28-year-old said: "What do I like about them? I like them better than Melbourne, that's for sure."
The four-time Melbourne best-and-fairest winner has started following GWS on Instagram, not Collingwood, and has clicked "unfollow" on the Demons' account.
Starcevich officially moved to West Coast on Wednesday morning, with Fremantle joining a three-way trade to get the deal over the line.
The Eagles gave picks 16 and 22 to Brisbane and 23 to Fremantle, while landing the Dockers' picks 12 and 33, with the Dockers also taking the Lions' pick 19.
Starcevich is one of three departures from last month's grand final-winning team, alongside Callum Ah Chee (set to join Adelaide) and Oscar McInerney (retirement).
"We're disappointed to see a player of Brandon's quality leave," Lions list manager Dom Ambrogio said.
"However, aside from the free agents we have targeted to bring in, our main aim in this trade period is to strengthen our draft hand, and we feel this trade has accomplished that."
With the Starcevich deal confirmed, Allen then joined the Lions as a restricted free agent.
Also on Wednesday, Fremantle secured defender Judd McVee from Melbourne for pick No.23.
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