I keep writing, but the longer I write, the more ideas I get to write about. If you're having trouble getting inspired, you can use any of these ideas for free. I tried to select the most interesting and collect them in one list
Fate
1. All the ideas about some characters from another worlds being summoned in holy grail war. Examples - Guts, Naruto, Adam (records of ragnarok), Technoblade and e.t.c.
2. Reincarnation in the past. Example - Reincarnated as one of Iskandar's soldiers, reincarnated as shurale (Creature from Tatar fairy tales), and e.t.c.
3. New master in holy grail war. Jyst like Fate of a Gamer (i forgot the name)
One piece
There will be my ideas about devil fruits and powers which your MC can use
(Fate) Drake powers as a devil fruit, lancer powers as a devil fruit (you can use any NP such as Rhongomyniad or Gae Bolr).
(LoL) here lives idea which i stole from a dead funfiction on another site. Pyke powers as devil fruit. Sett powers as fighting style (this 1 i didn't steal).
(KnY) Basycally, any breathing style as sword technique. Reincarnation as Mori's shadow, but you get blood demon art.
(JoJo) Any stand as a devi fruit.
My Hero Academia
here lives my best ideas which i even tryed to write, but gave up and gave them to you.
(As a teacher)
Timestop + unlimited knife supply. 2 versions. 1 You are an epic UA teacher in leather raincoat and masc covering your mouth, you are super edgy and super strong. 2 You are a Yaoyorozu family servant, which protects Momo.
8 Gates. You got no quirk, but you found out that you can concentrate your energy to "open gates" from naruto, or you can make that MC's quirk.
(As a student)
Basycally any anime power.
I really appriciate MGR powers here. Imagine Nimu try to kill you, and you stand like "Nanomachines, ugly abomination".
(As a villain/antyhero)
Also any anime power, but I really like Juzo Inui from No Guns Life. First - you are AllMight's oponent, but then you became antyhero/hero during USJ incident and 1shot nomu with Funke Faust.
I'll add ideas if I'll have them. if you need some help, I can try to help you with your fanfiction idieas.