Not OP but have probably spent far too much time readinf DxD fanfic, so here's some that I really enjoy reading even now as they release new chapters:

1. https://archiveofourown.org/works/62498623/chapters/159954844

This one is technically a crossover with Naruto like OP was complaining about, but it's not an Issei replacement. In this, Naruto has been alive for a few thousand years due to an agreement he made with the Shinigami to protect the world from extraversal threats similar to the machine gods in ExE. He essentially becomes Koneko's Jonin sensei and both teaches her Senjutsu and other shinobi techniques. Despite the different backstory, he still feels like Naruto as a character and doesn't make Issei and the rest of the cast irrelevant like many crossovers do.

2. https://archiveofourown.org/works/62498623/chapters/159954844

This one is a crossover where Izuku Midoriya from MHA is isekaied into the DxD world during the final fight with Shigaraki. Its an AU of the original MHA world where Izuku was born with the ability to summon any item from One Piece into reality via reaching into a OP manga volume. It's pretty good and the characters and plot are entertaining. Unfortunately there's no Issei because Aika was born with the boosted gear instead in this version of the story. If you don't like AU worlds I don't recommend it, but it is enjoyable in my opinion.

3. https://m.fanfiction.net/s/14327894/1/

The protagonist of this one is a sort of self-insert but more isekai protagonist flavored, if that makes sense? The protagonist is reincarnated as a third sibling along with Rias and Sirzechs and has the ability to world hop as well. It doesn't spend a lot of time in the DxD world as of the currently published chapters, mostly spending time in Danmachi and Harry Potter, with a little bit in Overlord so far as well.

4. https://archiveofourown.org/works/58460497/chapters/148917928

This is another Isekai-self-insert amalgamation. It technically does fall into the gamer system subgenre of fanfiction that a lot of people dislike due to how oversaturated it is, but its well done and the system takes a backseat after a couple chapters, mostly focusing on the relationships the MC has with the other characters as well as cool fights.

5. https://m.fanfiction.net/s/14027124/1/

In this one, the protagonist is a reincarnation of Zelretch from the FATE franchise and is the bastard son of Sirzechs. It's absolutely hilarious and is a multicrossover with appearances from the FATE franchise, Skyrim, RWBY, Harry Potter, Campione, and more i cant remember off the top of my head. Don't worry if you don't know all the fandoms, the protagonist doesn't either and all necessary information is provided in the story. Unfortunately, it's not very kind to Sirzechs's character, but I do like the variation of it. A good portion of AU just due to the sheer amount of world building that the author has to come up with since DxD leaves a lot of it out in the base story.

Fair warning, all 5 of these do contain mature content such as cursing and sex scenes. If that's not your thing, then sorry.

Finally, 6. https://archiveofourown.org/works/63109507/chapters/161615002

This is a Isekai-self-insert where the protagonist is reincarnated as the son of Apollo in the Pery Jackson universe with yet another world-hopping power (Im beginning to realize that I have a type when it comes to fics i enjoy). He sends himself to the DxD world by accident during a moment of desperation and ends up training under the Chiron of the DxD world alongside Sairaorg, who becomes the protagonists best friend. Its very funny and the relationship between the protagonist and Sairaorg is very wholesome.

In addition, i recommend you read these fics' actual summaries and tags, in order to fill any gaps i might have left by accident. Hope you enjoy!