Any film can be made good no matter what challenges it has. You can make a sequel to "Manos: Hands of Fate" and make it good. There's nothing stopping you except for maybe your imagination. To say a sequel or a new installment in a series could not be done well, even if it's eighteen years later, is to claim that everyone's imagination must be as limited as your own.
For the record I want to make it clear that I never said that a good sequel to the Indiana Jones trilogy couldn't be done. I for one have a personal motto that anything is possible. My feelings are that with the very uncreative slough of movies that are coming out of Hollywood today (hey, lets make movies out of the Dukes of Hazard and Miami Vice, and make another "Big Mama's House sequel everyone!) that I don't think that it is very likely that an Indiana Jones sequel will turn out very well because it will probably be treated only as a cash cow "franchise" and have a huge focus on special effects and little focus on story. Who knows, maybe the script writer they hired is a real ace and is going to blow me away with an awesome Indiana Jones 4 story that is so darn creative that I would have never thought of that approach for a million years. It would be great to be pleasantly surprised for a change! However, I am not going to hold my breath.
(ROTS being the most visually ambitious SW film)
Return of the Sith is visually stunning yes, but yet another example of the very weak story writing coming out of Hollywood these days. And before the flame posts start rolling in on how I could say such blasphemy against a Star Wars film, let me say that my opinion of ROTS story is not based on some geekish Star Wars universe detail that was missed but instead because the film failed to follow many fundamental story-writing techniques from film school 101, the most disappointing of which was an anti-climatic ending where Obi-wan strikes down Anakin on the lava world.
When you are storyboarding an action sequence, to raise the tension of a seen and capture the emotional arc of the combatants, you have to increase the amount of jeopardy that you have the hero is in during every progressing stage of the fight. Then, when it looks like all is lost for the hero and that he is surely going to die, by chance some unexpected miraculous event happens that allows the hero to gain the upper hand and win the fight in a harrowingly close victory. To see an example of this, lets look at the endings of two other Star Wars films: A New Hope and Return of the Jedi.
In A New Hope, you have a small band of rebel star fighters taking on a superior enemy, the Death Star. As soon as you hear a rebel pilot say, "Look at the size of that thing!" the situation looks grimmer and the suspense heightens. The Y-Wings all go in the trench for their bomb run, but get blown out stars before they even make it to the target. It was the Y-Wing group's ob to hit the Death Star's exhaust ports. Now they're dead. The situation grows grimmer for the heros, and the suspense heightens.
Now Red Leader and the first group of X-Wings goes in to attempt a bomb run. One by one they get picked off, and as they do the tension grows even higher. Now all that's left is the X-Wing group lead by an inexperienced bush pilot from the sticks named Skywalker. All of the battle-hardened pilots have already died? Once chance does this farm boy have? The situation grows still grimmer for the Rebel Alliance!
Now the Imperial TIE fighters are closing in on Luke and his X-Wing group. First, Luke's childhood friend Biggs gets incinerated by the fighters. Then Wedge's X-Wing gets knocked out of the fight. Now Luke is all alone, he is still not close enough to the target to fire, and Vader has him in his gunsight. The situation grows still grimmer for the heros, and the suspense heightens. Vader lets of his first salvo of laser bolts, effectively blowing R2-D2's head off. Now Luke truly is alone! And then it looks like it is all over for Luke when Vader says, "I have you now!"
But wait-- here comes that miracle that allows the hero to barely be able to win the fight that I was telling you about! Han Solo, who everyone thought was a scumbag for running with his money and not helping the rebels has miraculously showed up and saves Luke's butt, shocking the audience! Now Luke is clear to bomb the exhaust port and save the day! And what do you have here? A perfectly executed fight scene that has you on the edge of your seat the whole time, surprises you with the outcome, increases the jeopardy for the heros in every scene, and is so memerable that people still are moved by it 28 years after it was made! With Return of the Jedi you have the exact same thing-- Luke is getting his butt fried by the evil Emperor's force lightning, and is clearly helpless and suffering. And then, right before Luke is about to die the completely unexpected and miraculous event happens-- Lord Vader, the Emperor's trusted right hand man, suddenly turns on his master, throwing the Emperor down the Death Star's reactor core and saving his son's life.
Now let's look at the battle on the lava world at the end of Return of the Sith. Throughout the fight, despite the fact that we are told that Anakin's supposed to be one of the strongest of the Jedi, you don't really see any clear victor during Anakin and Obi-wan's chaotic struggle. Despite all of the foreshadowing that Anakin was going to be some ultimate bad guy, Obi-wan never seems to be at much of a disadvantage. Then they stop fighting all together, losing any tension from the seen that had built up in the scene, if any, and Obi-wan says how he has the higher ground, and if Anakin attacks him Anakin will lose. Then Anakin attacks him anyway and loses.
Lame! Where is the increase of jeopardy for Obi-wan? Where is Obi-wan certain to get defeated to make us get on the edge of are seats worrying about him? Where is the miraculous and unexpected twist of fate that allows Obi-wan to narrowly escape death and gain the upper hand? I'm sorry, but it's just not there! Obi-wan cuts down Anakin as easily as a food processor dices a head of lettuce, and doesn't even lose his breath while doing it. The control of tension for that scene was so poorly played out that I doubt Lucas even bothered to storyboard that sequence at all.
Here's the bottom line that I am trying to get at with all of this (and it's not me just trying to anger every Star Wars: ROTS fan out there-- if you liked that movie that's fine with me): You can have all of the greatest most top-notch and visually stunning special effects in the world, but if you don't have a good story to back it up, all you have is a special effects demo reel and not a feature film. The special effects are supposed to add strength to the emotional power of the story of a film, The story should be the film's strongest attribute and not just some excuse to showcase special effects.
So going back to what Christian said, yes you can make a great sequel to any movie under the sun if you spend a lot of hard work, time, and effort on it. And I would love to watch another Indiana Jones sequel if they do it well. But I want to see one-hundred and some odd minutes of fantastic storytelling, and not just one-hundred and some odd minutes of Indiana Jones being surrounded by dazzeling CG creations and things blowing up with no rhyme or reason for it.