Shares in Deutsche Telekom plunged nearly 5% on Wednesday. T-Mobile US stock slid 3.5% in afternoon trading. Investors recoiled from reports of early-stage talks for a full combination between the German giant and its U.S. majority-owned unit. The proposed structure? A new holding company. It would launch an all-stock bid for shares in both firms. Existing shareholders would own the entity, potentially listed on exchanges in the U.S. and Europe. Deutsche Telekom holds 53% of T-Mobile already. The merged group could top $300 billion in market value, eclipsing China Mobile as the world’s largest wireless operator with over 200 million subscribers. Scale like that packs firepower. Morgan Stanley analysts point out it could fuel further acquisitions across borders. T-Mobile has morphed into Deutsche Telekom’s growth engine. The U.S. market outpaces stagnant Europe, where debt-burdened carriers battle fierce competition. But markets hate uncertainty.…