As a cosplayer, you suffer a constant temptation. There's a fabric sale, over there's a shoe sale, here is some sewing stuff, and patterns....the problem is often that you have to decide one way or the other what characters you want to cosplay. But how?
To me, this question is very important. I follow many cosplayers on deviantart, Pinterest, Instagram, plus, I'm at conventions, so I get to see many, many different costumes and characters. I often come across characters I don't even know but I still think "Wowser, could imagine doing it myself".
Rule no. 1: The look
I'm a visual guy and character design is my first criteria and the first thing that draws me to a character. And for now, it doesn't matter whether I know that character or not! I have (had) costumes on my lists where I still have never seen the anime series, let alone still don't know the character's persona. But the character design was appealing, the clothing design interesting and cool and the overall look challenging but reqarding when done correctly. Not all costumes (I mentioned it) meeting this criteria will eventually make it to the top list. There are many characters I like at first sight, but after reviewing them, I decide to drop the plans. For example Envy from Fullmetal Alchemist (I'm still not sure about him). The design is interesting, no doubt, but after some weeks, I (for now) decided to not cosplay him. Same for Masquerade from Bakugan, Ray from Beyblade or Tsuna from "Katekyo Hitman Reborn".
But if it meets criteria 1, then there' still
Rule no. 2: The charakter, orthe show
Yeah, the costume can look cool as hell, but if the character is a jerk, I don't want to cosplay him/her. I'm not saying that this is the only reasons costumes got dropped in this round. Merle from Escaflowne for example has a cute design, but she's kind of a nag. Same for Sailor Mars. Another K.O. criteria is the show itself. The character design and costume for Ryuko Matoi from Kill la Kill is great and interesting for both crossplay and genderbent, but after only one episode of this show I knew: it's not worth it! I still love seeing pictures of these costumes, but to me it went "in the can" because of the - to me, your opinion will most likely differ - pea-brained and hyped-up show.
Another subcriteria is the character's behaviour or...let's say: gadgets. Celty Sturluson from Durarara got a check mark because she's a motorcycle babe (and I love motorcycles), and D.N.Angel's Dark Mousy is a woeful thief but, come on....WINGS!!!! DAYUUUUUM! I'M IN!
Now, we have two critieria defined. But there is another point which eventually lead to many costumes being dropped that were consistent up to this point.
Rule no. 3: Hello? It's Me!
Because cosplay is more than just wearing a "gorgeous costume" (bought or made). You have to embody the character. Not the proportions, weight or gender. I'm talking about playing the role. Whether I can play it convincingly or not. Whether I feel comfortable playing it or not. And this criteria was the coffin nail to many costumes. Sephiroth for example is a totally cool character with a first class design, but I could never portray him convincingly. And Haseo from .hack//G.U. or Ichika Orimura from Infinite Stratos are showing their belly, which I don't feel comfortable with (at least not with these characters). Okay, you might argue "Then do a different design, dude", but to me, it's not the way I would want to portray them, not my standard. And that's why Envy will eventually be kicked, too.
Of course, you may have other criteria to choose your costume. Currency of the show, recognition, support or pursuation by friends... and each of these criteria are just as valid. I personally have the above mentioned criteria, but sitll, some characters are bypassing it (Mulan would have failed at Rule 3, but my wife Kari decided it would be a funny thing, so why not?).
But yeah, that's basically how I choose a character.