After review of latest MCU boards and possibilities I believe a "XM Next Technology" device called 7800 XN could be practical.

The following features are possible:

- Stereo Surround (for every game) -> already works
- Wireless Joystick feature (controls full console, too) -> soon available
- HDMI output (fine tuning required)
- Screenshot mode, result is stored on SD Card. Organized as screenshot and highscore feature.
- Library function (store your cartridge to SD-Card and run from it)
- USB/Bluetooth support
- Price tag at about $100

Disadvantage it could only be done as internal mod not external (due to missing expansion port and not all signals are available on cartridge slot).

In implementing screenshot/highscore feature all games benefit from implementation, e.g. it is not required having new games as it works independant from 6502. You push the button on your wireless controller and the system stops the game, save screen to SD-Card and you are able to enter name/info. The SD card can be used on PC to copy the pictures.

The library function offer the possibility to insert a cartridge, switch on your console while holding the select button and a menu on screen offer to play a game from sdcard or allow to store current cartridge to sd-card.
this might work with 7800 and 2600 titles (may be not with all of them as it depend on bank swichting and pokey support).

Theoretically a pokey/yamaha emulation is possible but I have not verified yet if it is easy to implement.

As opposite to Atari XM extension it is internally, does not really provide features for "new" Atari 7800 XM dedicated games and is more dedicated to existing software base.

Good or bad idea?