Cracks fingers*
My kind of topic! (Fired up)
There are many tactics regarding the formation of ideas. For some, it just comes easy to be imaginative. Around my circle of friends, I enjoy joking around and doing one man skits (voices included). By doing improvisational plays and even songs, I have become quite adept at story making.
#1- Practice. There more you do something, the better you become. Try making improv songs, or just improv rap (easier). Alternatively, try and provide your friends or family with jokes/ witty jokes. Doing so increases idea generation and creativity. It won't be easy, I promise. I wanted to be the "funny" guy and the "leader" of the group. Took me from 6th Grade till I graduated to get there (the funny part).
Have you ever truly watched an anime and made note of all the literary devices (symbolism, archetypes, etc...)? Simply re-watching it will make them blatantly obvious. There more you read books, light novels, visual novels, watch film and anime with plots, and play games with a story, the more adept you become at seeing these things and not only knowing how they are used but also why. The art that is telling a story is beautiful, complicated, and fun. So being exposed to all these stories will in turn give you more exposure, sub-consciously or directly affecting your writing (Ex. I didn't know THIS archetype existed. Oh wow, what symbolism. Hmm... this is quite the monologue).
#2-Comprehend Literature. Take an AP, IB, or College course on Literature, or just be exposed enough and discuss until you understand the (can't believe I'm saying this) Language of Literature. Because just as you analyze these works for what they are both superficially (work as a whole) and internally (the many components), doing so with your ideas will make you want to change some things or even add more. This will sometimes cause a chain reaction in the evolution of your idea/ story.
This next one is direct and simple. But trust me, it works. It's also something you see people do a lot when analyzing a piece (Anime and film are types of literature. True aficionados discuss anime in high detail because they do this and unintentionally TIP #2).
#3-Ask "why". Keep doing this, and every event, character, even setting will have meaning. From the time of day, to why the chair is blue.
Many authors of many works do this (myself included). You don't necessarily have to start from the beginning.
#4-In Medias Res. Start from the middle, hell, start from the goddamn climax! It's fun as hell, and working backwards is not hard at all, as you know the destination, so just work your way back. (This helps with planning plot twists.)
Sometimes you just can't get it right. It's okay to know point A and point B but not the line in between. We can see a clear example of this with the author for "Bleach". cough cough*
#5-Make it up as you go.
There is almost always a source of inspiration and influence (whether it be subconscious or direct). Examples include "Goku-> Naruto", "Bible"-> "Evangelion", "The 7 Samurai"-> "Samurai 7", Raptor Jesus-> Jesus Christ (no offense intended).
#6-Be inspired! Whether you adapt a work (with permission, or it has become old enough to be a public work) or draw inspiration for elements in your tale, go ahead. Hell, if your story was just one massive "What if...?" scenario of a previous work, but with different characters and setting, as long as the audience like it, go ahead.
Don't limit yourself. I wanted to stay in a specific genre. As I began making the story for my manga, I wanted to keep it Vanilla Shonen [happy ending]. (In progress from the age of 6. No, I'm not kidding lol, seriously. This thing's seen an unhealthy amount of revisions.) But now, it has become a partial deconstruction (see Evangelion). I also decided to take some characters/ situations that didn't fit in that I liked out. And just like that, more stories formed! I know have a total of 5-6 manga in planning (still a long way before artistically ready)! A Shonen [Deconstruction], Romance Mecha [Right!? But very unlikely, at least not soon], Romantic Comedy, Shonen Adventure Comedy Hybrid [Vanilla], Horror, and Adventure [Sengoku Era]! These were all created because I left my comfort zone. And you know what, I love them all!
#7-Fuck your comfort zone! Trust me.
I hope this helped. Please excuse my language as I'm quite passionate. There would normally be more emotion but this was already long as is. Still, my apologies. If you have any questions, or anyone else, feel free to ask (confused, clarifications, explanations, etc...). But most importantly, be open to everything. I'm not %100 correct. This is MY mind. What works for you may very. So be sure to value other inputs.