Well I would suggest for the most part Nuso Esva's forces yes with the remainder made up of pirates. For the pirates you could reuse some of the Hutt ships and troops from the earlier versions minus the bloodscars obviously. As for Esva's forces the descriptions of the Firekilns and smaller support ships give at least an outline for their appearance and overall capabilities. His fighters are described in more detail than the rest of the units so that could be something to start on. You all have done such incredible work on the EotH ships and vehicles from scratch I'm sure you could create these units.
There are a few main problems with doing some Unknown Regions/ Anti-EotH factions like that. Like Slornie said, in Nuso Esva's case there's really nothing to work off of, same goes for the Vagaari, the other main known contender of the Unknown Regions. Designing ships is a lot harder and takes a lot more time than working off some kind of concept, that's why the EotH have taken so long. Sometimes it's just impossible to think of a good design. For example, the Asendancy was around the second EotH ship to have its redesign, however it took me almost a year to get a new design I actually liked, and our hard drives are littered with renders of earlier versions of every single EotH design. There's something like 12 alternative Phalanx bridge area designs that came up. There's also the same issues with land units.
I guess the main point is that even if a faction isn't playable, it takes just as much work. If you look at any normal RTS, the typical setup is 3 factions with about 10 units each, for one single game mode, which can take a mod a long time to do. Minor faction units take just as much time and effort as a regular playable faction, as sometimes more. Especially if it's meant to be the main enemy in a GC.
As far as pirate/Hutt units go, they'd get demolished in no time by any EotH ship. This is part of the reason the Hutts got cut in the first place; waste of a faction slot and had no discernable impact on the game.
The thing is I know it would take lots of time and effort and I don't wish to add to your burdens if I can avoid it.
I was just thinking that if you had certain scenarios where you could only play as one faction without having to deal with the other two Main factions it could help people get a feel for that faction and encourage them to try said faction in the larger scenarios since they would be more knowledgeable about it. Few people know too much about the Hand, I myself find them fascinating and hope for Zhan to write more about them but most people know only the IR and NR well.
Well really if we have to do something gimmicky like that it partially means we haven't done our jobs and made them interesting to people in the first place, and if they don't want to use the EotH because of their obscurity/lack of development in general, they'd probably just ignore that GC since it'd be the EotH against another even less developed and interesting faction. I mean, it's certainly an idea that would work for it's own purposes if it were to become feasible, however as a vehicle for growing interest in the EotH I think it faces the same issues as the EotH on their own do. Any information you find on them would still be useful in case it becomes more doable in the future, however they should probably get their own thread. In general I like to keep it to one topic per thread in case anyone has anything to add without the discussions going 5 different ways in one thread.
NR could have a scenario with them vs the Yevetha with about a dozen worlds and the IR could have a scenario with them vs the Warlords with about 18 worlds or so. This way each player could feel out each faction on their own without having to wage a multitude of wars on multiple major factions with enormous resources. Kind of training grounds so to speak.
This is the general thought process behind the GCs that have been added since the initial release, starting with Zsinj's although Zsin's is being reworked to the point that it'll probably be more of a conventional era GC. Into the Cluster and Empires at War were both designed as small "bite-sized" GCs centered on essentially single-arena wars. The Yevethan conflict is definitely high on the list for future GCs.
Also, the Royal Guard units you suggested have been done. This is why you shouldn't trust me when I say something is low or high priority.