I am writing an Evangelion/Super Smash Bros. crossover fic called Super Smash Evangelion or I may change the title, actually and I don't know if I should turn this into a fic that's like a full television series, like a big story mode game, or just a fic shorter than them.

Honestly, just start writing it. You can outline it, see what method fits your needs and wants better, but you'll do better to start writing chunks of it and see what happens. When you put it down anything (doesn't matter if its a sentence or several pages), it'll give you a more concrete object to work with.

Exactly right. That's how I started my one and only fic. I had the urge to write a scene; I thought about it, wrote it, and then found that another scene wanted to follow it. After about half a dozen scenes I realised that I might be able to finish things, and produced a very rough outline of the rest - simply a list of scenes and two or three exchanges of words I didn't want to forget to put in. I was then able to follow that through, though it got a bit bulked out as I went on because I realised that I'd missed stages that needed to happen. (You can watch the process in the discussion linked in my sig - offering the pieces to the community as I went was a valuable way of getting feedback and encouragement, I found.)