"I want sushi."
不思議 is an entirely different word according to Jisho.org. Duolingo accepts 寿司が欲しいです, but I didn't try 寿司が欲しい, so I don't know about even more informal forms. In general, Duo accepts formal and neutral forms. The mods have said that they try to accept more informal forms too, but the review process is difficult.