Having reread a few sites discussing what little we know of Lucas’s ideas, I think it’s safe to say the Whills would serve as the main protagonists and antagonists. Based off the only Lucas canon – the OT, the PT, and TCW – the force is best understood as divided into the light and dark sides. The Jedi were not perfect but there is nothing from Lucas that suggests they need widespread reform.
I saw this concept art picture of Darth Talon, with a shadowy figure hovering behind her (https://miro.medium.com/max/2063/1*-ez8L1awc_mE6G4kac6Gwg.jpeg). The caption by the concept artist described the shadowy figure behind her as the puppeteer manipulating her. I think it would be interesting if we made this an Evil Whill. Maybe Darth Plagueis found a way to secretly preserve his own consciousness after death through the Midi-Chlorians as the Jedi had. He searched and found a Force Sensitive Twi-lek with attractive features and manipulated her to seduce the Jedi Killer and turn him against Luke.
We can drop the Plagueis idea if you’d like, but I think we’d still need to explain where this Evil Whill came from.
So I think we would learn that there is a Dark Whill somewhere that has brought evil into the galaxy, cheesy as it may be, and this being was what corrupted Sidious in the first place. Keeping in mind this would remain a Skywalker / Solo drama, I think Ben Solo may be the Dark Whill’s “next target” and start corrupting him to restart the Sith, and it will be up to Luke’s daughter to redeem her cousin and break the cycle.
Ben Solo would be Sam Solo. In George’s trilogy, he wasn’t Force sensitive. However, the Jedi Killer left a real impact on Luke, so I’m toying with the idea of him being Luke’s son.
I don’t know if Luke should have an heir though. You’d need to explain where his wife is, and if he changed that rule in the Jedi Order, and as you said there’s no evidence that George wanted the Jedi way to be changed. If Kira was Luke’s daughter, than we would have to go down the route that Luke doesn’t know and she was hidden as a child by her Mother who was in trouble and is now deceased.
This is a very good base plan. At this point, I don’t think there is much more we can do without creating our own story out of things we would like to see.
In that vein, I will make a few suggestions. Trying to keep in the Lucas Lore but all my ideas are really fanfiction at this point.
– Darth Talon would make for an interesting antagonist, but I don’t think she should live past Episode 8. Perhaps Jedi Killer kills her in an act similar to Snoke’s death in TLJ. If the Dark Whill / Plaguies remains the big bad it won’t be as crazy as the DT… it would be more Jedi Killer promoting himself to directly serving the Dark Whill.
– If we go the Sam Solo route, I’d prefer to make it so that Sam thinks he doesn’t have the force because he doesn’t have a natural affinity to it like Kira. But perhaps in the 9th episode, he realizes he had it all along, he just needed to believe he had it in the first place.
– Jedi Killer could work as Luke’s very first apprentice, sorta like Kyp Durron in the EU. Luke sees on him as living proof of his early failures, from before his academy was properly established.
– Why not make Kira’s mother a mystery? Luke never tells her who her mother is, and she spends a while wondering who it might be. It could another Jedi (inspired by Mara Jade) in hiding that returns by Episode 9.