The few fics I have read where Luke is unable to convert Vader, Luke usually ends up dying to the Emperor shortly afterwards. I would like to read some fics where Luke actually survives and is not converted to the Dark Side himself and has to contend/reflect on this failure.
I can't remember from whom or where it was from but I vaguely recall anecdote or story about a child (perhaps the son/daughter of a dictator) who grew up in a loving household coming to realize that their parent was an absolutely terrible person when it came to everyone else.
I'm not looking for that direct tale, but I do want to read some stories with a similar theme regarding this topic.
When I was reading the TV Tropes page for Early Installment Weirdness for Star Wars there was quite a bit of details, especially under the Expanded Universe section, that caught my eye. For those who don't know, "Early Installment Weirdnesss" is basically the experimental stage when a series starts off when the creators are still setting the tone and picking and choosing what does and doesn't work for their story.
EarlyInstallmentWeirdness / Star Wars - TV Tropes
Personally the concept I found most interesting was the notion that masters of the Force don't carry lightsabers. Now I don't hate that Yoda and Palpatine use lightsabers, but I truly feel it would have done them a better service of establishing how strong they are if Lucasfilms had got more creative in how they would wield the Force during their duel in ROTS. I wouldn't say the concept is entirely gone from the franchise, with Palpatine's force storm in TROS and Luke's Force Projection in TLJ, but I think at the very least the prequels could have been more interesting if for example the masters of the Jedi Council were shown to have established expertise in different areas of the Force to the point where they rarely have to draw their lightsabers as a method of combat.
On a side note, the further idea that only fallen Jedi who defected to the Sith kept lightsabers and "true" Sith never wield them could have added fascinating tidbit of cultural worldbuilding.
I would also love to see fics where the more "average" Separatists realize their movement is being manipulated by Count Dooku/Sidious and the CIS Corporate Leaders and try to move away from them, perhaps leading them back into the Republic as a result.