I know the guys at spark really well
The Teensy 3.1 is super powerful processor I also know Paul and have talked to him about doing this.
For me, honestly the next version doing color screen and faster processor etc. etc. It all comes down to external code execution. I'd really like to be able to store all the games on an SD card and allow an operating system to execute them.
It's probably possible to develop something with an ARM 4 or maybe even a 3.... But the hardware of an ARM 7 will support it natively. But at that point, we're just building an open source GBA. Is there interest in that? We will see.
I love these discussions and actually, that's why arduboy is so simple because as soon as you try to add more features it's easy to get carried away with the complexity.