T-Mobile has something of a reputation for not always getting the latest smartphones, as they don't have the iPhone, there's no word on the HTC One X or Galaxy SIII, and their current best option is probably the One S or older Galaxy SII. You can always use international GSM devices as John has pointed out, but often people like to have carrier branded versions.
However, today a leak is suggesting a new middle high end device from Samsung will be coming to T-Mobile sometime soon. Found by TmoNews, the leak comes in the form of hidden user agent string that shows a Samsung device with a 1280 x 720 display, ARM11 processor, and model number T699. There is no information beyond that, but what we have already tells us quite a lot. First of all, as we've already seen model numbers for both a T-Mobile Galaxy Note and Galaxy S III, this can't be either of those. As far as 720p Samsung devices go, that really leaves the Galaxy Nexus as the only option.
It is a little weird that T-Mobile would be getting the Galaxy Nexus so late in the game, especially because you can use the unlocked Nexus from Google Play with T-Mobile, and everything including HSPA+ will work properly. The only possible advantage is that T-Mobile would then subsidize the cost of the phone, but with the unlocked version at $399 that doesn't make too much sense either.
Still, I personally like the idea of having the same device on all carriers so that consumers aren't stuck to one carrier if they want a certain phone, so even though it might be a little late, a T-Mobile Galaxy Nexus still wouldn't be a bad thing. Anyways, even if the T699 isn't the Galaxy Nexus, a new Samsung device with a 1280 x 720 display on T-Mobile wouldn't hurt either.