I am going to build a board, that fits into the space where the circuitboard of the old Nokia 3210 sits. It will come with the same layout for buttons and LEDs. The screen will be replaced by a OLED display that is much more beautiful than the original dual color TFT display.
For the CPU the current design is build around the STM32F439 but I am not sure if I want to stay with the STM part. I favour the i.MX7 controller since I am currently designing a board with the conroller anyway.
Furthermore I am looking into WiFi and Bluetooth. I am thinking about using the WiLink Modules from TI (http://www.ti.com/product/wl1807mod) The current design comes with an ESP8266 Module but that does not provide Bluetooth.