Unfortunately, Sins of a Solar Empire: Rebellion, the new standalone expansion for the game, doesn't add a campaign -- although it does still allow for players to skirmish against the AI, a prospect made alternatively too easy or too hard to be fun by the game's difficulty settings. Still, this is a multiplayer-focused game, so (despite some tantalizing, story-driven cutscenes), the lack of a campaign is no huge letdown.
In Rebellion, Ironclad Games legitimately buffs up Sins of a Solar Empire's graphical engine, enhancing visual effects and allowing for more stuff to happen smoothly on the screen at once. Rebellion also adds many worthwhile tweaks, including three "new" factions (more on that in a second). Also new are the Corvette and Titan class ships. Titans are gigantic, superweapon-esque battle platforms that can single-handedly take on flotillas of enemy vessels. They also have powerful "buffs" they can impart to friendly ships, making your side able to take and dish out more damage (and look much more intimidating).
Corvettes, on the other hand, fill the fleet role between the workhorse frigates and the large cruisers. Corvettes are small and generally require less research to unlock than cruisers, but offer more versatility than frigates. They're also unique to each faction and tend to feature "debuffs" to enemy abilities rather than direct damage: the Advent Rebels, for example, use their Vespa-class corvette to knife in among enemy ships and damage their shield recharge rate, whereas the TEC and Vasari Loyalists have corvettes designed to stop enemies from engaging their long-range engines and escaping from a battle. These new ships add a yet another layer of complexity to fleet combat in Rebellion that makes it as in-depth as any chess match, and are much more interesting to would-be tacticians than the honkin'-big Titans.
Now, you may be wondering what I mean by "Loyalist" and "Rebel." Here's how it works: the three "new" factions included in the expansion aren't really new at all: they're just "rebel" versions of the three original factions. Ostensibly, these rebel versions have a new raison d'etre that differs from their original clan (the Vasari Rebels, for example, are more willing to co-operate with humans than their loyalist brothers), but since there's no story or campaign, that all ends up being flavor text in the game setup screen. Otherwise, Rebel and Loyalist versions have a lot more in common than not: Corvettes and Titans are unique to each, but every other ship type is shared, and they all look and sound the same, too. You do get a few faction-specific technologies for each (these often tie in somehow to their general ethos – the defensive-minded TEC Loyalists can build two starbases in a single gravity well, for example).* So, the reality is that the rebel factions ought more properly to be termed "tweaked" than "new". Advent players who'd rather focus on combat and internal improvement, as opposed to culture bombing and influencing pirates, would choose the Advent Rebel faction instead of the Advent Loyalists, for example, but would still basically be playing Advent.
And there's the rub, really. For the most part, Rebellion offers tweaks: graphical tweaks, faction tweaks, technology balancing, etc. it doesn't give you a lot of honest-to-goodness new content. As a result, when you play through it (at least as a veteran of the series), it feels thin, especially at the $40 price. Part of this feeling of thinness was inevitable: because Sins of a Solar Empire has been painstakingly balanced, through patches and expansions, over its life cycle, and because adding things to the mix always screws up that balance, Stardock's development team is put in a tough position. Do they go for new and fancy and risk breaking the perfect formula, or do they play it safe and hope players will think the new-ish content is worth the money? Rebellion proves they've unequivocally chosen the latter. There's nothing basically wrong with that, of course, but it feels more "patch-y" than "expansion-y." Then again, Rebellion does give you access to all the content on the previous two expansions, Entrenchment and Diplomacy, so starbases, planetary defenses, and diplomatic victory are all here. Much more worthwhile if you're new to the series.
