I nvever worked it out in detail but have a solid idea of what the group is about. I also came up with it before 4B, so the different villains teaming up then didn't happen in Once yet at that moment.
The members of the Misthaven Five (Mary, Jersey, Sun, Facilier and Arachne) all would have a "tragic" backstory that drove them into doing underground business. In the EF they were the alternate option to Rumple for making deals, but because Rumple always was the first choice they were kinda forgotten by everyone.
They resided both curses because they resided in a pocket dimension of Facilier from the shadows. But they eventually would cross over and start an underground maffia in SB and eventually take over the whole place, mostly duo to Mary's powers.
In the end our heroes would go to their base in SB and defeat each villain (in a different fashion to the QoD. Like they first encounter Sun for example, and some stay behind to fight him while the rest continues to stop Mary, who is the main antagonist of the arc).