So I've seen this one thrown around a bit, a third faction made up of underworld ships, Scum is the vernacular. Though I don't generally hear too heavily what ships would be involved, so I thought it might be worth discussing what the baseline stuff would be needed, and what Canon/semi-canon ships could fill the roles. Three main sources I can think of: Trade Federation(pre-CIS, they sold everything if the price was right), SWEAWForces of Corruption faction is the largest reference of semi-canon ships to pull from, and of course general EU modern and legacy. Now I had one other train of thought on this before we hit the lists. With the introduction of campaigns, and the nefarious nature of the Underworld, an additional way to add some depth would be for ships to have a second higher mercenary cost, now in the campaigns you could add troubled worlds that are controlled by the underworld. When the Rebels or Empire take control, they could recruit mercenaries for a higher point cost, but outside of the general resource cot perhaps. These worlds would be incapable of holding shipyards.
So, first Ship area I'll throw in on is Trade Fed. Seems likely that the Hutts could have bought a number of pre-clone wars Lucrehulk Droid control ships as an insurance policy along with wings of Vulture droids, likely retrofitted with additional turbo-laser batteries, but more for intimidation than fighting the empire straight on. It'd be what a medium or large base? It's been a while since I've checked the stats and I'm still not perfect on the game scale.
Now, I could just ramble off a list of Zann Consortium ships from SWEAWFOC, but how many would actually be viable, and what would be useful, I think I'm better off leaving that to you guys. Along with the new canon I don't know of many new additions to the underworld, so again I leave that for further instruction, and I'll try to do my best. Oh, and a pack of Uglys is an obvious option
So we've got our Fighter packs, Tiny, Small, medium and Large ship slots to fill up, lets give this a go