Huh, I felt I was confident to construct a basic sentence but now I'm sort of confused...
So, "たいです" replacing the masu-stem indicated desire? Neat, hadn't encountered that yet.
-たいです expresses "want", yes. So 食べたいです is "I want to eat." 寝たいです is "I want to sleep."
If you don't want to just say "
I want to see
<character> in Smash Brothers."
「スマブラに
<キャラ>を見たいです。」
You could instead say something like 「
<キャラ>はスマブラに出るといいですね。」
"It would be great if
<character> appeared in Smash Brothers."
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So, since it's to Masahiro Inoue aka Kamen Rider Decade, guess I'll go with DCDはスマブラの中を見たいです!Went with the abbreviations since I'll be drawing this since I don't think US Miiverse has Japanese support, and I wanna save space.
I wonder though what the Japanese typically use when requesting characters...
Whoa, whoa, slow down. You had the particles right in your first attempt;it should be 「スマブラに」not「を」. And you don't need 「の中」.
Ex:
DCDをスマブラ
に見たいです。
or スマブラにDCDを見たいです。
Note, I changed the 「は」 to an 「を」.  I had it right in the Mario example but I got a bit confused with all these edits I'm doing.
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In response to this part:
I wonder though what the Japanese typically use when requesting characters...
I've found you a couple examples:
Safe for work, don't worry:
http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44232108
Third comment down:
アイスクライマーちゃん可愛いですよね!
スマブラに出るといいですね!
The person is saying Ice Climber-chan is cute, and that it would be great if she appeared in Smash Brothers.
And this tweet:
https://twitter.com/ashi_4589/statuses/435182322363551744
クロムさんスマブラに出るといいな〜
The person is saying it would be great if a character named "Kuromu" appeared in Smash Brothers.
Note: I'm not saying this is the only grammatically correct way to request characters in something; it's just a commonly used one.