According to a report from Bloomberg, the DoJ has apparently told T-Mobile and Sprint that they will need to create a new competitor to themselves if they are hoping that the deal will go through. This competitor will not actually be owned by either company, but the carriers will need to “lay the groundwork” that would allow a new carrier to emerge and pose as a potential challenger.
One of the concerns of the deal is that it would result in the telco industry slowly becoming a monopoly. At the moment, there are four major carriers, but with T-Mobile and Sprint merging, it would mean one less carrier which also means less options for consumers. Whether or not the carriers will agree to these conditions remains to be seen, but apparently talks between the carriers and the DoJ have been productive.
It will be interesting to see what kind of new competition could rise from this, and also a bit ironic should that newly-created carrier end up being a bigger success.