Евгений Лубяной

type IItem = {type: 'IItem'};
type IDataSet = {type: 'IDataSet'};
export type IAgentList = {
    [key: string]: IAgentBase<any>;
}

export interface IAgents extends IAgentList {
    items: IItemsParser;
    page: IPageParser;
    parser: IWebParser;
}

export interface IAgentBase<T> {
    parse(): Promise<T>;
}

export interface IItemsParser extends IAgentBase<IItem[]> {
}

export interface IPageParser extends IAgentBase<IDataSet[]> {
}

export interface IWebParser extends IAgentBase<any> {
}

export interface IAgentsResult<T> {
    items?: IItem[];
    page?: IDataSet[];
    parser?: any;
}

type ResolveResult<T extends IAgentList> = {
    [key in keyof T]: T[key] extends IAgentBase<infer R> ? R : never;
}

// в реальности все иначе, написал для понимания желаемого результата
async function runAgents<T extends IAgentList>(agents: T): Promise<ResolveResult<T>> {
    const result: ResolveResult<T> = {} as any;
    Object.keys(agents).forEach(
      async key => result[key as keyof T] = await agents[key as keyof T].parse()
    );
    return result;
}

async function main() {
    const agents : IAgents = {} as any;
    const result = await runAgents(agents)
    result // typed
}