They will most likely dump what remains of the CDMA network, absorb Sprint's spectrum (pending anti-trust/regulatory certification; luckily, in this environment, I don't think they'll be forced to divest spectrum to AT&T or Verizon), and offer 5G nationwide to compete against wired broadband providers (with the mobile comms market saturated, growth opportunities are needed to stoke growth).

It's a good deal now that Softbank gave up on keeping control of the combined org and is satisfied with a minority stake.

Citations:

https://www.reddit.com/r/tmobile/comments/5usle9/exclusive_s...

https://allfor5g.com/ (combined Sprint/TMO 5G info microsite)

Disclosure: T-Mobile customer for 18+ years, no other relation