in order to achieve my initial plans I have an idea for an initial prototype:
Arduino Pro Micro - main controller. Handles button input via analog pins, and a Bluetooth dongle via softwareserial pins. An i2c master (for lcd display and communication with i2c slave devices below). Also adds HID capability.
ESP8266 - allows wifi options; connecting to networks, AP mode, webserver.
20x4 LCD Display - i2c controlled, for showing menus and feedback from slave devices
Arduino Nano - i2c slave, for expansion. Pins to be broken out and made available via the cartridge socket on the GBA case. Eventually cartridges can be made to work with this breakout.
This combination for components, along with a well coded set of menus, should provide me with a great starting point for this project.
I have two GBA cases to start filling; on for the above layout, and one featuring a Nextion touch screen. Replacing the lcd with this will mean defining an extra software serial on the Prother Micro, but allows me to add a full colour GUI to this potentially great device.