I have a suggestion for T-Mobile to work on that would be an excellent "UnCarrier" move. Work for a way to incorporate WiFi calling on unlocked Android smartphones. I know that there are many technicalities involved, but I hear that T-Mobile's WiFi calling will most likely become available for iOS 8 devices when they are released. Android currently has 52% of US smartphone market share, while Apple's iOS has 42%. Developing a way to install this feature on unlocked Android phones would be a huge draw for new customers! This could be even be introduced as a feature that is purchased separately from the plan, as a one time fee, or as a small monthly charge for a period of time to be added to monthly charges. These charges could help offset the software development costs. This would be another service that no other US carrier provides and open up a new level of "freedom" that no one else could compete against.
With Android's open source nature, perhaps there is a way to incorporate the necessary internal OS elements needed to allow the installation of a T-Mobile WiFi Calling Dialer app (apk). This is something that has been discussed on such forum sites such as XDA Developers and other tech forums for years. T-Mobile did in the past (2011) release IMS source code to the Android community but I haven't seen any further push from T-Mobile in this area of development (IMS is the method used by T-Mobile to implement WiFi calling).
In my personal experience, I think that your service and value is the best by far out of the "Big 4" nationwide cellular Carriers. After suffering for years under unreasonably expensive Verizon contracts, and other sub-par prepaid options like Virgin Mobile and their almost unusable Sprint based data network, I switched to T-Mobile prepaid. I have been a customer for 2 years using a Samsung Galaxy SII and in that time I have saved over $1300 (including the upfront cost of the phone) compared to what Verizon would have cost me, and I easily get 10 to 20 times the data speeds that I got on Virgin Mobile, and I get those speeds on T-Mobile with HSPA+, not even LTE! I personally would love to purchase an unlocked phone such as the Moto G 4G LTE, a OnePlus One, or a Nexus 5, but Wifi Calling is currently not available on those devices. Being able to add the feature after I activated the phone is something that I and I'm sure many others would be willing to pay for.