Detective/mystery works. Suits and detective-style costumes are standard.
Cosplay from detective/mystery works typically features suits and detective styles. Serious expressions and props (magnifying glass, notebook) are used to recreate 'solving a mystery' scenes.
Detective Conan, The Kindaichi Case Files, and The Devotion of Suspect X are easy to try with suits + simple props.
Sherlock, In/Spectre, and The Apothecary Diaries create a world view with period costumes and character-specific decorations.
