Finally they came from the shadows...?
At this point... the natural way for Oda to introduce these guys, if he's going to use them, is though Cross Guild, right?
Cross Guild is very attractive to Bounty Hunter/Underworld types because it allows them to get work and work under a powerful umbrella, without having to sell themselves out fully. Like, the New Giant Pirates just straight up left.
Blackbeard wants allegiance; he'll let you do what you want, to a certain extent, but only if you're powerful.
Cross Guild gives Underworld operatives a huge means of making money without sacrificing autonomy.