For an underworld major faction to work, it needs what every major faction needs.
Threads and posts, lots of IC activity.
I don't think the map as advertising for the faction will draw will be nearly enough attention or activity required to maintain Major Faction Status. The threads needed keep a major faction running healthy and that develop are better advertising than any amount of hexes. Stories that get our writers excited to write underworld characters, and stories that other factions would be interested in collaborating with, with the OOC coordination to match. Yes, that means most of the time we'd probably be playing rent-a-villain, but at least going into it with that mentality sets expectations.
The issue with underworld factions just by virtue of the characters is the individualism that comes with the aesthetic. Drumming up shared goals and interweaving stories requires a lot of mutual buy-in, and the idea of catching "everybody not a Jedi, Sith, Mando, or imperial" in one big faction sounds nice on paper, but the playing the map game for the sake of it seems to be a fast track to burnout without the reward writing underworld stories.
TLDR if we can't make it work as a minor first, we're doomed if we shoot for major.
My proposed solution: something involving UWC. The unifying factor of most underworld types is greed, we have a system that generates 'wealth' via activity, I think making use of it would fit thematically and could serve as a unifying focus to keep different types of criminals writing together.
Also, if we want to run a Hutt empire, we need more Hutt characters to come out of retirement, all's I'm sayin