First and foremost, just going to say for fandoms that I do Canon x OC roleplays for the most part but I ALWAYS double for whatever partners that ask to roleplay fandoms with me. So where they'd roleplay as Canon for my OC, I would roleplay as a Canon for their own chosen OC or canon so long as it's a M x F pairing.
Anyway, onto the topic itself, the fandoms I typically have difficulty finding partners for are the following:
Hakuouki: Demon of the Fleeting Blossom
Attack on Titan
Pokemon
Final Fantasy VIII
Gundam Wing
Hakuouki, I'd get potential takers but the plot would never get off the ground to actually roleplay in it. I have a character that depending on the Canon they're against is either a swordsmith's daughter and apprentice OR a sibling seeking revenge against those that killed her sister. Mind you, the latter one she would have already gotten revenge for her sister and I wouldn't have written in gorey details as to what she did to them in the roleplay depending on partner's comfort zone. If anything, this fandom I get possible takers but then it never happens OR more recently on another site, someone told me that my character's BACKSTORY wasn't inclusive...when I said in my message that it was a backstory and that if they wanted for their OC to be mine's friend/rival/family/whatever that I was perfectly willing to make changes regardless of how much work it would take and that the backstory was not part of the main plot for the fandom that we were going to roleplay in. Like...I just don't get that person's logic. But anyway, onto the next fandom.
Attack on Titan, I had 2 roleplays going for it but would end up with people that only replied once every month essentially. Where that doesn't bother me too much it's kind of annoying after a while when I have it mentioned that 2 weeks is how far my patience extended to. Both eventually just stopped replying altogether which is fine but I still want to do this. Not tired of writing this yet. What I wanted to do in this was a Levi x with my OC.
Pokemon, I'm also not sure why this one seems cursed either since it's another popular fandom. Basically, all I want is a Blue Oak [Obviously aged up using his Alola age which would be in his 20s] x OC roleplay with my OC. I'll get some really good ones going only for them to just die out a few posts in. I had 1 good one going but reasonable stuff happened so why it died out was understandable. This roleplay I have in mind can take a few different routes so writing it here would be a chore. But it's mostly meant to be comedic/lighthearted with some serious elements added into the mix. As for what Pokemon regions I would be well versed in it would be Kanto/Johto/Alola/Hisui. Used to know Sinnoh and Hoenn but for some reason now, I can't really recall these games much and it's sad.
Final Fantasy VIII, I had three incidents with this one. 1 person just wanted to rp stuff that was in my NOs in my RT so that didn't work out. Then there was someone who legit had a Gary Stu character where everyone knew his OC and loved him. Except for Seifer [who had to be the only one who was how he should be character wise]. And he somehow was able to summon every SINGLE GF or Guardian Force where all of them communicated with him like a friend, had their own personalities etc [imo I don't mind having a GF be more interactive with a character but this was ridiculous when he essentially had an auditorium mind full of these beings in his head]. And was so selfish with these GFs that when I went to use one specifically for my character, he'd keep trying to control it so it had some connection with his character. He also made his character like...super strong. Needless to say, I finally said something about it all and the dude ghosted me after that lol. Lastly, someone essentially did a surprise, doing all of the NOs in your RT during a sex scene and left lol. But anyway, all I wanted to do was do an honest rp in that setting with my OC. Depending on the canon they were roleplaying against one would become a Sorceress unexpectedly as once dormant powers suddenly woke up OR another had a more comedic side of things where she was the sole survivor from a mission where a man in his twenties killed her entire team that she was with and left her with a phobia of men [ridiculous as it might sound but it's meant for comedic purposes]. I'd say more but it would make this post too long lol.
Gundam Wing, I had 1 really good roleplay that I enjoyed the story of but the person could no longer rp it anymore for understandable reasons. Since then, I've not found a rp partner that clicked. I was looking to do OC x OC with an original concept in the fandom setting. The only real canon stuff included would have been Earth, the colonies in space and the Gundams. What was so interesting about this one is we had 3 world powers at war with one another on Earth. 1 of them had a wealth in Gundamium which made the other two go after them for it. We roleplayed a small group in each faction to show a side to their end of the story so we could see and explore why they were doing what they were doing. Alternating after so many posts before switching to the other group. I'd like to do something like that again but haven't found anyone that clicks with me for it. That and I guess some are intimidated by the fact that we'd be rping a set group of characters at a time. From my experience, not many people are confident enough to handle multiple characters at once in an rp. I had multiple too in this rp so it's not like 1 person had all of the characters. Taken now
But yeah, that's what's been my issue with these fandoms.