The main argument against T-Mobile and Sprint's proposed merger has been the possibility of reduced competition; after all, Canada only has three major carriers, and that hasn't worked out great for the country. The U.S. Justice Department will reportedly only approve the merger if the combined company will sell off enough wireless spectrum for a new independent carrier to function, and the latest company to show interest is Dish Network.
According to Bloomberg, cable company Dish Network is in talks to buy both Boost Mobile (Sprint's NVMO brand) and wireless spectrum owned by T-Mobile and Sprint. The purchase would allow T-Mobile and Sprint to move forward with a merger, as approval from the Justice Department is one of the few roadblocks left. Amazon was reportedly also interested, but it's not clear if that is still the case.
Ironically, T-Mobile has been a harsh critic of Dish's spectrum holdings over the years. T-Mobile CEO John Legere criticized Dish as early as this year, saying in a tweet, "They've warehoused $11B of spectrum for years and missed every build deadline." The company also wrote a letter to the FCC demanding that Dish's spectrum should be relicensed.
Dish has floated the possibility of building a 5G network using its existing holdings, but the company has also been looking for someone to buy its ~100 MHz of spectrum entirely. However, major carriers are likely no longer interested in what Dish owns, especially after the FCC's recent auctions.