Futaba and L will develop an Odd Friendship
Both are highly eccentric geniuses who effortlessly deduced the identities of the protagonists. Futaba's Alibaba alias may have even assisted L in one of his previous cases. Perhaps they unwittingly meet up and hit it off well and Futaba is able to leverage that later to get L to either back off or assist the Phantom Thieves in taking down The Conspiracy. If the fic covers the Laboratory of Despair Arc...
Light will be a counselor for prisoners in Maruki's reality, and his biggest success stories will include many of the criminals he killed during his tenure as Kira. Ren will have to work extra hard and delicately to snap him out of his delusion, and debate with himself whether it's the right thing.Also, Ryuk will either be rendered an Un-person until Light reawakens, or be the only other person besides Ren and Akechi to notice something wrong with reality.
L will gain a Persona of his own and join the Phantom Thieves out-right.
L will become a Palace ruler
Think about it. L isn’t exactly known to be a paragon. In canon, he’s been described as “slightly evil” and is more than willing to do anything to solve a case. He also admits himself that he’s childish and hates losing. It’s entirely possible that, with the bitterness that comes from learning the Phantom Thieves took down his first Worthy Opponent in a while before he could that his heart ends up distorting and he gains a Palace. This results in the Thieves having to work on a time limit before he just barges in and arrests them, not caring about needing definitive proof to convict Light anymore, as well as giving Light a new reason to find his real name so they can access his Palace.It’s also possible that he ends up taking Sae’s place, being the antagonist of a Palace themed around Envy that leads to Ren’s arrest (notice how there’s no flash forward to her interrogating him when he kickstarts Light’s Confidant?).
Also, given that he’s Light’s Foil, it would only make sense that, as Light ends up back on the path of justice, L ends up straying away from it.
Why Yaldaboth hates the Shinigami so much.
Ryuk has made it very clear that the Holy Grail does not like Shinigami to the point that even an Extreme Punishment would be a Mercy Kill. I think the reason for this is simple. It sees itself as the God of Control, and what is the one thing everyone wants control of but can never have? Death. The shinigami are the ones in control of when people die, and Yadly sees them as a threat and representative of everything it cannot be. Thus it has a burning hatred for the Gods of Death and I imagine the first few Shinigami who dared to go to the Metaverse or Mementos were quick to not make that mistake again