King_Disturb
you should've stuck with one piece for a while, then with fairy tale you as you were doing already, just continue the cannon just making a few differences here and there, until some big change happens due to the butterfly effect caused by the presence of MC. Carry only the story by adding you own scenes, then find an end in the fairy tail world. transport MC to another world again with a similar magic system, but more powerful. this way he will get more powerful to get to his goal.
I think I understand where you are coming from. I belive the reason why you are not able to see/picture where the story will is because the problems for most characters are mostly solved if not explored in some way. This might be the reason so I think it woupd be best, that when/if you return to the series, to have some characters have more underlying problems like try writing from the view of some of the lesser known guild members and maybe even make some up to help get a better idea of where thos story could go. Also adding more changes to the story and canon should help as well, like having some people naturally awaken haki now that that power has been introduced to this world. Or even introduced characters that went through also were isekaied twice and ypu can come up woth a reason for why such things are happening. Such as the reality of the fairy tail universe is breaking apart so it is using people from diffrent realites to fix these misong bits of reality but it needs to those pieces to be strong enough first. Try find reasons for the unexplained parts of the story and explore them, because they might inspire you. Also you don't have to listen to me or do anything I suggested, these ideas just came to my mind while reading this "chapter" so I thought I should mention them. P.S. Take a break till your ready for a new novel/fan-fic or returning to one, because otherwise the story won't reach its full potential.
It is perfectly understandable, most novels that feature reincarnation with an ROB, or just reincarnating in anime worlds generally end up on permanent HIATUS. Although it still has potential to continue. This particular fanfic felt like it started losing it's touch due to MCs character built around revenge, which he accomplished and Fairy Tail was basically him settling down in somewhere. It would have likely been difficult making him transfer to yet another world without him wanting desperately to return. Ans it would be even more difficult to make him the centre of attention without hogging the spotlight from Lucy and the rest of the cast and, if the original MC from the anime was someone like Issei from DxD it would be quite easy, but here is Fairy Tail and most if not all characters, even some villains are quite likable. My personal sugggestion is to first somehow have him free himself from the ROB in a way that isn't disappointing, since that would be the biggest problem going forward. Perhaps have the FT world not be a world not be anything that the ROB can properly predict and One Piece was a proper world and ROB sent him to it's past and gave MC knowledge of that world and made him think that it was anime, so that MC could grow strong quickly, so he could be sent to FT for his enjoyment. For getting rid of the ROB, maybe have Ankhselan get involved and help MC due to the ROB messing with his worlds life (maybe interfering with the worlds reincarnation, or MC messed with life, but the blame ends up being placed on ROB). As for the story, make him replace Macarov and slowly fade into the background. Just let him be around for events that should have included Macarov. Like when Magnolia got Isekaid he managed to protect himself, or have him play a big role during the demon arc, since Haki isnt magic and should be very usefull in those situations. Write the fanfic centering around the main cast but simply include him and change some things. My primary idea is to have him teach Lucy and maybe Wendy Haki and just write the story if they had Haki.