I'm guessing T-Mobile will start upgrading EDGE to HSPA+/LTE and on top of that, deploy their new 700 MHz LTE (we may need new phones
) and buy up more spectrum. This is their biggest complaint from their customers and T-Mobile is growing, they need to expand.
T-Mobile's CTO Neville Ray did say that T-Mobile will perform expansions at an interview after the Un-Carrier 4.0 announcement, but he said that T-Mobile is looking to build more towers (which is good because in my house, I only get EDGE and outside, I only get one bar of LTE and I live in Philadelphia).
But on the other hand, upgrading thousands of towers to LTE will take billions of dollars, plus the fiber-backhaul needed and workers to complete this. Plus, building a cell site costs $40,000 to $250,000, which is a lot.
But as always, T-Mobile will eventually become #1 and their LTE network will be kicking butt, but let us also hope that Sprint/SoftBank do not buy T-Mobile, otherwise we would be screwed.