T-Mobile has some serious coverage gaps in rural areas like highway stretches and small villages throughout the US, and a merger with Sprint could help to solve those. Network research firm Mosaik put together coverage maps for the sub-gigahertz spectrum holdings of both T-Mobile and Sprint, and when the two were put together, the map revealed that most of T-Mobile's rural coverage gaps were filled, and the combined carriers boasted heavy spectrum coverage, on the order of 21 to 80 megahertz, throughout most of the US, with some hotspots having upwards of 80 megahertz, and a scant few places having no coverage at all.
In these maps, the spectrum being looked at falls under the gigahertz barrier, meaning that Sprint's massive cache of high-band spectrum from its old iDEN and WiMAX networks are not even considered. With those added in, the two carriers combined could easily deploy the full gamut of currently planned 5G solutions almost nationwide. Fixed wireless and small cell coverage could be provided by Sprint's high-band spectrum, while T-Mobile's lower-band spectrum could provide the longer-range connections that transmit data processed over the higher-band connections, or could be their own independent 5G connections, with the right software and hardware. Should a merger not happen, the latter approach is how T-Mobile is planning its 5G rollout, while Sprint is planning the former.
Talk of a merger between Sprint and T-Mobile make it seem all but certain that the two will join forces in the near future, and likely take on a 5G rollout together, using each other's spectrum as described above. Thus far, there has been no official confirmation of a merger happening, but both companies have been in talks over the possibility. If the merger does happen, of course, the above scenario is far from the only way that the combined carriers could choose to deploy a joint 5G network; it's merely the simplest way. Taking advantage of crosstalk between spectrum bands, achieving total spectrum dominance through aggregation and the use of unlicensed LTE bands, dedicating certain spectrum bands to certain use cases, and many other possibilities are all on the table if the two decide to go through with this merger.