The answer to this question would vary depending on how much effort you want to put into dressing up (also known as Cosplaying in the anime world).
For example, if you wanted to throw something quick and simple, you can do what I did in my profile picture, and dress up as Ryu from Street Fighters. All you really need is a karate uniform (or taekwondo, or maybe even judo) and a red bandana. You can then just cut off the sleeves and make it jagged-looking, and voila! Simple enough right?
For more complicated characters, you may need to do a lot more work (ie, buy colored contact lens to match your character). You may also need to sew in designs for clothing, get a wig, etc.
Overall, for Halloween, it would probably be the best to dress more mainstream. Imagine spending 20+ hours into making a costume, just so that only 1 or 2 people (out of the 100 people you meet) recognize the character.
Have fun this year during Halloween! :)