Just throwing in my two cents on a few of the topics discussed here.
Fett - stop the Fett fan service. He's menacing in ESB because he doesn't say much and doesn't take any crap. He even questions Vader in the carbonite chamber (ballsy, isn't he?) There's nothing wrong with him never being seen in ROTJ because he made his delivery to Jabba, collected his money, and he's off chasing down some other poor schmuck.
Force ghosts - I read all of the posts and agree with many of the attempts to explain it all. It just hit me, Obi Wan says, "You can't win, Darth. If you strike me down I will become more powerful than you can possibly imagine." He has learned a deeper connection to the Force than Anakin ever did (hence Vader's surprise). We see lots of jedi bodies in the PT because they never escalated to that level. Even those on the Jedi Council hadn't figured it out. Yoda did because Yoda's amazing. Obi-Wan figured it out through meditation and discipline in his years in hiding on Tatooine. Maybe he also took midicholrian injections [ducks and covers]. If you leave out the whole nonsense about "teaching Obi-Wan to commune with Qui-Gon's ghost" in ROTS, it's a plausible explanation. I don't think Anakin deserves a force ghost at the end of ROTJ, but it does give it a nice happy ending.
I have no problem with Fett being in ROTJ, as long as he doesn't go out like a punk - something I believe Ady has said he'll be addressing. You can see it both ways. Some might want it unresolved and that makes the story darker and more cynical - if there's no justice, what else do we have in store for our characters? Others might feel that there does need to be a conclusion to his story rather than it be left without him getting his comeuppance. It was afterall going to be called 'Revenge of the Jedi'. Luke should take vengeance for his friend.
I'm on the fence. I don't care one way or the other about Fett. His role is always going to be perfunctory for me.
The fan-service didn't start in earnest until Attack of the Clones and that's where the main problems lie IMO. I was all for seeing the Mandalorian armour in the Prequels - ideally as the main antagonistic force, but not specifically in conjunction with Fett. But instead the movie went full fan-service - showing him with his daddy, which shrunk the universe.
In the remade prequels of my imagination, the armour would look a bit different to what we see Fett wear in ESB. The helmet would be the same design, plus a few things like the chest and shoulder/knee pads, however there'd be additional parts of the armour we would assume were lost/discarded when Fett found a set of it. Likewise, some parts of Fett's armour (like the jumpsuit, hair braids, etc) would be absent from what we see in the Prequels. This would give Fett his own cobbled together identity, and would suggest that his armour was largely scavenged.
Regarding Force ghosts - I can see both sides of whether Jedi should disappear upon death in the Prequels. I hated the hamfisted way George tried making it a whole new skill in ROTS, but at the same time, I see your point that Darth does seem surprised when Ben does it in front of him. The ideal would be if it was established in the Prequels that only a few, extremely powerful Jedi had ever achieved 'enlightenment' and learnt to ascend in this way upon death, and it hadn't happened in hundreds of years. That way, Anakin can have knowledge of this ancient skill, and therefore be surprised his old master has managed it, rather than it being entirely unknown to him.
Of course, it would be difficult, if not impossible to incorporate this info into an edit, as we're extremely limited by the source material. The only convo I can think of where Anakin discusses an ancient power is in the convo with Palps in ROTS about the power to stop people from dying (and later he even refers to it rather perfectly as "the power to cheat death", albeit in his terrible monster voice). If this convo could be somehow made to be about the power to 'ascend', whether it's being discussed with Palps, or if the convo could be made to happen with another character, then a prior knowledge of what happens with Ben in ANH could be established. It would be ironic if Anakin turned to the dark side in order to try to become immortal, but it was the Jedi teachings that held the secret all along!
As for Anakin having a ghost in ROTJ, he absolutely should. It's part of his journey, and for him to be redeemed and standing there at the end is the conclusion of his character arc. When you say he doesn't "deserve" a ghost, well once you take out the butchering and child murder from the Prequels (a given, I think), Vader isn't quite so unforgivable. Yes, we know from ANH that he's a bad guy who was instrumental in hunting down the Jedi, but that doesn't mean we need to see him as a violent murderer.