A long while ago (on a blog quite, quite close), I made a series of posts looking over aspects of 4.0; this covered the characters, a brief summary of the KotFE story, and just general universal changes.
Things I never got around to covering included the Star Fortresses and the Alliance system. Whilst the latter will only be written when the final Alliance Alert of this season is released, I see no reason to delay the summary of the Star Fortresses.
The Star Fortresses are the 'Flashpoints' of 4.0, but they are an "impure form". They aren't part of the Group Finder and even its harder mode can be done entirely by oneself. This is seen as a good thing by people who like having more things to do by themselves, whilst others see wasted opportunity. I personally don't mind them not being in Group Finder as they haven't really resonated as "true" Flashpoints to me due to the personal aspect of the larger story which they're a part of, but I can understand why people are upset that they aren't there.
The only known Star Fortresses so far are the ones above Alderaan, Belsavis, Bothawui, Hoth, Nar Shaddaa, Tatooine, and Voss. The one above Bothawui is only mentioned in the story preceding your visiting one Star Fortress of your choosing, whilst the others can all be visited.
I would describe each Star Fortress as a "less random" version of Kuat Drive Yards. Indeed, only the layout, Bonus Objective, and two of four Paladins are randomised; the bosses and overall objectives remain exactly the same; each Fortress does have its own unique final boss, however. The small randomness is a nice touch, as it means that not everything is the same, but not to the extent that this difference may make the overall thing more tedious. The only exception to this is that a couple of the Paladins can hurt on Heroic Mode.
The main differences between Heroic mode and Standard mode is that your Alliance Specialists have (somehow...) managed to smuggle dead-drops around the Star Fortress which can only be interacted with if you yourself are at Influence 10 with them and have a special buff which can be picked up at Influence Level 5 right at the start, and the Exarch is forced into doing battle with you because they have no more shield to hide behind. The final fight with the Exarch is long and requires hopping from platform-to-platform to deactivate containment fields for the Sun Generator beforehand, and the Exarch is really quite stun and knockback-happy...
Speaking of the Exarchs, they've changed considerably over Arcann's reign. You first encounter them accompanying him to your cell on Arcann's flagship, before Koth then appropriates a shuttle from a whole troop of them a couple of Chapters (and five whole years) later. They don't seem anything special, but at the end of Chapter IX, they have lightsaber pikes and shields instead of blasters, and Theron even states that Senya wouldn't want to cross one ever. Either Koth is a far better soldier than anyone gives him credit for and Senya's a complete wuss, or having their shuttle stolen by Koth caused the Exarchs to really put some effort into reshaping their image.
The story behind the Fortresses is relatively simple. Theron is angered over the Bothawui Star Fortress going "lesser Death Star" over Bothawui and resolves to destroy any others which remain. Annoyingly, the only one to actually do anything is that above Bothawui, and we never get to see it destroyed.
The problem is, nobody knows how to go about destroying it. Therefore, the logical thing to do is infiltrate one and see how it works on the inside. Considering that we visit Zakuul several times and even take place in its most famous arena contest despite being its most notorious criminal, this isn't strangely as out-of-place as it would otherwise seem. Upon fighting to the control room of a droid known as the EPHEMERIS, the commanding officer - an Exarch - informs us that our efforts are futile as its shield cannot be deactivated from within.
This then leads to our rendezvousing with six Resistance Fighters, one on each world, for whom we gather supplies before leading the charge into the nearby shield generator. These six individuals range from "interesting" to "tediously forgettable"; there's Choza Raabat, an actual Ithorian Jedi from Alderaan, K'krohl, a Whiphid Mercenary Recruiter from Belsavis, Hemdil Tre, an Arcona War Historian from Hoth, Veeroa Denz, a Nautolan Sith from Nar Shaddaa, "Deadeye" Leyta, a Selonian Mercenary from Tatooine, and Rokuss, a Gormak from Voss.
Veeroa and Choza are probably the most interesting characters here, but unfortunately for people who like her, Veeroa is all-too-likely to leave of her own accord once Zakuul is defeated, as she is mentioned to enjoy her life in the shadows. At the less positive end of the scale, Hemdil doesn't shut up at all, making him almost as bad as Treek, and Rokuss is a walking diplomacy disaster zone. Last thing we heard, the Voss absolutely forbade any Gormak to leave the surface (notably setting up a confrontation during the Inquisitor storyline) and Rokuss leaves with his own band of fellow Gormak soldiers. Maybe it works this time around because the Alliance has the support of the Voss themselves and they can keep an eye on the Gormak on Odessen, but it's still infuriatingly ambiguous.
Destroying the Star Fortress above each of their worlds will, of course, unlock the respective Resistance Fighter as a Companion.
As much as making each of these characters an alien who doesn't speak Basic is obviously an attempt to reduce voice-acting costs, I do really like finally seeing some of these species actually be Companions. The most prominent example being Choza; we've never ever seen an Ithorian Jedi fight in this game before (the only notable one before this simply wore a robe and criticised a youngster for his typical 'youngster' impatience on Tython), and now we have one as a Companion, although the length of his neck means that he often leaves you wanting for personal space...
Not to mention that four throats has an annoying tendency to cause incredibly bad breath.
Leyta being a Selonian from Tatooine makes little sense, though; it would have made more sense for the Fortress to be posited above Corellia, as this planet has the Republic shipyards and also happens to be the first planet where you encounter Selonians. You can raise this point in-game as well; there's just nothing on Tatooine to make putting a Star Fortress above it worthwhile. Arcann's really quite silly when you think about him.
I still say we should have had a Kel Dor somewhere along the line as one of these Companions. Unfortunately he - although I'd love for them to introduce a female of the species in-game at some point - would probably be as aggravating as Hemdil.
In fact, I'd even go so far as to say that Choza and Veeroa are two of the most refreshing Companions they've ever included in the game, simply because they are the first male true-Jedi and female true-Sith Companions, respectively. Guss, obviously, doesn't have the mindset of a Jedi whilst Dark Jaesa is not only a former Jedi, but she's acting in a way that somebody would believe a really, really Dark person would act like and completely overdoes it. Veeroa, at least, received proper training at the Sith Academy...
Overall, the Star Fortresses are... decent. They don't hold much of a candle to proper Flashpoints, and in my mind they don't deserve to. They do have some nice drops including the various Decorations, and the Resistance Fighters are quite fun to interact with, but there's not all the much special about them otherwise.
That said, destroying a Star Fortress does result in a pretty awesome escape cutscene which is very reminiscent of the second Death Star's destruction in Return of the Jedi.