Mapped typeとConditional typeを使ってclass/interfaceのプロパティの型を変更するサンプル ref: https://qiita.com/kurogelee/items/a6955d2f399d3929ef03

file2.txt

{
    id: string;
    id2?: number;
    key: string;
    value?: boolean;
    names?: Array<string>;
    names2: Array<string>;
    value2?: Array<Array<string>>;
}

main.ts

type OrFunction<T> = T | ((val: string) => T);
interface Content {
    id: string;
    id2?: number;
    key: OrFunction<string>;
    value?: OrFunction<boolean>;
    names?: Array<OrFunction<string>>;
    names2: Array<OrFunction<string>>;
    value2?: Array<Array<OrFunction<string>>>;
}

sample.ts

type Plain<T> = { [P in keyof T]: ExcludeArray<T[P], (val: any) => any> };
type ExcludeArray<T, U> = T extends Array<infer R> ? Array<ExcludeArray2<R, U>> : Exclude<T, U>;
type ExcludeArray2<T, U> = T extends Array<infer R> ? Array<Exclude<R, U>> : Exclude<T, U>;