It's tricky to know as they don't publish what phones will be coming to the network and certainly don't let on future prices. If you had a specific device in mind you could call customer services and ask if and when it will be available, but otherwise we are all stuck with the information they provide.
Unless somebody on this forum has insider information....?
(edit) Actually, look at this link:
http://www.t-mobile.co.uk/shop/pay-m...=ON_QM_A_BuyAT
The Deal 6 shows the Nokia 6700 coming soon. That's going to be a good phone, a proper successor to the 6300. It wont have all the bells and whistles of modern media phones, but it will be a damn good phone. As above though, no idea on price.