Thus what was wrong with the Star Wars Prequels (and Return of the Jedi even) in the first place. It was all about cool stuff happening at a mile per minute with no regard to telling a good story.
Except he forgot to put in the cool stuff.
Unless you mean John Williams' score...
what else can Lucas do for a trilogy set after ROTJ?
Here's one approach: extrapolate Anakin's story as an ongoing curse involving his kids. Vader can die, but the dark side doesn't die with him - it can't. It's eternal. This would involve rewriting the ending of ROTJ, but since I only liked the first half of that movie anyway, that's ok by me.
Instead of being off in Ewok-land, Leia needs to be aboard the new Death Star at the end - any why wouldn't she be captured at that point, since Vader must have figured out her identity pretty quickly after realizing that the twins survived? A daughter with the right looks adopted at exactly the right time by one of Padme's chief allies, gee, nothing suspicious about that. (I'd also loose the new Death Star angle - and place the final scene somewhere else - and ratchet back or lose the Ewoks as well, but that's just a bonus.)
Vader attacks the Emperor to save Luke. The Emperor fatally wounds Vader. Luke has the chance to kill the Emperor but realizing it's a trap to send him to the dark side, hesitates. But Leia doesn't - brushing off Luke's warnings, she shoots the Emperor, who dies realizing that he's gotten what he wanted - a chance for the dark side to continue its ascendancy, because now Leia has unwittingly "cursed" herself.
To Luke, the Force is a real thing with dangerous power, and its rules must be respected, especially by someone who is Force sensititve. To Leia, it's irrelevant mumbo jumbo and if you have a bastard like the Emperor in your blaster sights, you shoot. She respects that Luke believes in the Force, but that doesn't mean she does.
At first, the sequel trilogy seems triumphant. They're back on Coruscant, the remnants of the Empire are being defeated. As one of the leaders of the Rebellion, Leia has serious political power, maybe becomes Chancellor. But by now it's common knowledge that Luke is Vader's son, and even though he's trusted by the people who know him in the Rebellion, the vast majority of people aren't going to be so open-minded after the trauma they've experienced.
I would play up the angle that the Force is considered an actual religion - one that is now considered too dangerous to allow to exist. The only sure way to get rid of any possibility of the Sith is to get rid of the Jedi, too. So this becomes the policy of the new Republic, over Leia's objections.
The upshot is that Luke has to scram if he doesn't want to end up imprisoned, regardless of his sister's power. I'd have Han accompany Luke (at Leia's insistence) because Han is the right sort of guy to have along if you're trying to evade capture and survive by your wits.
There would be a parallel story of Luke and Han off having crazy ass adventures trying to evade the authorities - as well as Luke's attempt to find fellow Jedi (or anyone who is Force sensitive) and restore the Order in defiance of all prejudices against them.
Meanwhile back on Coruscant, Leia becomes increasingly annoyed and frustrated by the limitations of having to run a revived Republic which is going to have the same problems that the old one did, plus she's incensed at the ingrateful treatment that Luke has received and of course if it were ever known she's also Force sensitive, she'd be in real trouble. And if Vader could figure it out, so could her enemies.
The key here is that both the twins should have stresses heaped on them that makes it possible but not inevitable that they would turn to the dark side. Not knowing the outcome of this situation will keep audiences interested in the story throughout the three movies.
This gives the sequel trilogy a central dramatic engine - will history repeat itself with one (or both?) of the twins, and if so, which one? If they don't use this as the dramatic engine, I can't envision a story that would work instead. An external enemy doesn't really seem Star Warsy to me. (Granted, maybe I should read the Thrawn trilogy one of these days and see how well that works.)
This all comes from my theory that Star Wars is most Star Wars-y when it's based on fairy tales and mythology - the Emperor's curse that survives his death. If you veer from that basis (like the PT did, which had no mythical feel at all), then what you've got isn't Star Wars to me anymore. Unlike Star Trek, which is highly flexible and can embrace all sorts of stories, Star Wars seems to me like it's got to walk a very narrow line in order to maintain its unique identity.
