I think the problem here is that you have a series of connected 'ideas' that you're trying to force into one narrative which makes the natural character interaction, the SoL, flat.
When you write I'd almost recommend taking the idea or concept from where you start and plan from there using characters, their personalities, and drives to move the story and character interaction. It might take you somewhere else than you initially planned to go, but the story will flow more naturally. That includes for people that aren't necessarily part of the main cast, such as villains or side characters too.
Fact is, DxD's already pretty fleshed out with its characters, so it shouldn't be hard to do, and not hard to come up with some ideas about details about some of the less fleshed out characters' personalities if you decide to use them so long as they're faithful to the details DxD already presented, and it gives a better result than trying to force the plot from point a to b and forcing the actual character interaction around it like a cheap wrapper and I don't want to be rude but that's what your comment feels like.
I'd suggest, honestly, starting from the beginning with your concept, come up with some details and give yourself a rough idea of what you want from the initial arc of the story- not it as a whole- and start from the beginnig and make a rough outline of events you'd think would happen. Then, as you write and develop the characters and interactions as they should happen naturally and if that leads you away from your original plot but still feels like that's the natural progression then go for it.
For example, let's say you want to have naruto interact with Rias's group and have him at Kuoh academy. Alright, so you've set up the board but you think about it as you write Naruto- the school bores him and he starts ditching but he hangs out with Issei some. BUT- you think about it, Rias and her group would'nt really care about naruto ditching, but Sona would. So you find yourself writing a lto of ineraction with Sona because as Naruto adjusts and hangs out with Issei, he keeps pissing off Sona by being a delinquent and she's the student council president.
Now, you've set a timeline for youself beforei n your rough ideas that These events drawing him to Rias's group would happen because you want him dealing with Rias's group, but looking at the chemistry, it almost feels more like it would be more fitting that Naruto, with increased conflict with Sona, decided to prank her or something in response, but ends up accidentally finding out she's a devil which leads to a plot where Naruto's got to be convinced to keep quiet about the devils thing or maybe it was all a plot that was cooked up by Sona because she's got plans and is really smart.
Does that mean you have to drop the plot about naruto interacting with Rias's group? No, because you've already got him with a link to them with Issei, that is if the changes in issei by knowing naruto aren't enough to keep him from his date with death. Hell, maybe Naruto's presence there/arrival to Dxd's universe is enough that Azazel's interest is more active and he intervenes when Raynare is screwing the pooch and- as Issei and Azazel actually get along pretty well in canon and should be able to here- he recruits Issei or just saves him and acts as his buddy. Issei not being killed changes DxD events and maybe Rias complains to Sona (because honestly there's no way that a fallen angel appearing and dating Issei is outside of their knowledge considering she appeared in the heart of their territory while they're around- in the school) and complains about the total ruined opportunity and bothers Sona with an idea to help her get Issei because she's not so good on recruiting her own peerage because of the good luck she had in getting her own. Maybe Naruto hears that- with his planned prank- and we've got a conflict with him against them because Issei's his buddy so decides to get more involved to look out for his friend's best interests because Issei is a perv who could be tempted by a cute girl like Rias really easily.
See- still ends up with interaction, but it doesn't have to keep super close to canon at all times so long as the spirit and well... character of the characters is kept true to as you write. For example, these changes do nothing to affect the fact Rias is still engaged to Riser, which is the big motivator for her to find a solution to her problem one way or another.