It's completely standalone, so you don't need the original Trinity. If you have Trinity anyway, I remember hearing about a discount upgrade price, but I don't recall if anything came of that. They're talking about doing a collector's edition with a neat Kol Battleship model and some posters and stuff... and if you've already bought Rebellion when the Collector's Edition comes out, there will be a discount upgrade price.
Anyway, my favorite changes/additions:
TEC Loyalists: Each of the three races has two factions. The only faction I've tried for any meaningful amount of time... because it fits my play style so perfectly. It allows you to build two starbases per gravity well, reduces the resources required to upgrade them, and also grants combat bonuses to being in friendly territory.
TITANS. By far my favorite addition (aside from the factions themselves). Each faction gets its own unique Titan. You're only allowed to have one at a time, and they take a relatively long time/large amount of resources to get out, but each one is designed to be a beast on the battlefield. The AIs seem hesitant to deploy them. The one AI I've seen actually fielding a Titan immediately ran away from mine, despite it being higher lvl and in friendly territory (which also means some of my abilities were weaker). I later got him to commit it to battle, where I promptly ate it for breakfast. About 20 minutes after that another AI wiped him out. TEC Loyalists get the Ankylon titan. It's got abilities that protect nearby friendlies, debuff nearby hostiles, and increase armor repair/damage output for the titan itself. The Ankylon also has three passive abilities, which I'm assuming it shares with all other Titans, as they're fairly generic: Weapon upgrade, Armor upgrade, and Antimatter upgrade. Each level you get two ability points, and each ability can reach up to level 4.
Corvettes! They're small, cheap, fast frigates that act like fighters. They swoop in, attack, then swoop out. Each faction gets their own type. TEC Loyalists get the Shriken corvette, which sabotages the engines of ships as a passive on its normal attacks.