A few years ago, T-Mobile's parent company Deutsche Telekom (DT) was anxious to unload its 65% share in the US carrier, and one of the primary suitors was Sprint's parent company Softbank. Things are different now. Sources claim SoftBank is floating the idea of selling its 83% stake in Sprint to DT and letting the carriers merge that way.
SoftBank has been frustrated with Sprint's performance in recent years. It once saw a T-Mobile acquisition as a way to become a big player in the US, but talks were called off in 2014 with opposition from regulators. Since then, T-Mobile has taken over the number three carrier spot from Sprint and DT has changed its mind about the US market. It is reportedly no longer willing to part with its controlling share of Tmo. Thus, selling Sprint might be the only way forward for SoftBank.
With DT in control of both carriers, a merger should favor T-Mobile's management and business model. I'm sure John Legere will be happy. Any deal would probably still face regulatory scrutiny, though the Trump administration could be more accepting than Obama's was. AT&T and Verizon would more than likely come out against the deal, though. Sprint has a ton of spectrum that could make the new, larger T-Mobile a serious threat to the bigger carriers. The deal can't be officially discussed until April when the current spectrum auction is complete.