nesdev.org

If a perfect Gameboy flashcart were to exist, it must have the following :

SD or CF card support
Save to card
Menus with subdirectories
True MBC1/2/3/5 emulation
Real Time Clock Support
Rumble Pak Support

The cart should be able to completely hide itself from the Gameboy and game once the game has loaded. This is necessary to counter games that try to detect copiers. The cart should also not be able to interfere with SGB enhancement detection or CGB palettes for DMG games.

Nice things to have would be support for Digimon MMM01 and Kirby's Tilt 'n Tunble MCB7. I do not believe flash ram or eeprom support is necessary for non-Japanese games.

So get on it.

The problem with SGB detection like you say is the issue because the SGB is already running when the Menu comes up which is sees as not a SGB program. You can't avoid this as far as I know, other than having the game in memory on powerup like it should be. And I agree with kyuusaku, go for it. Don't expect anyone else to do it for you.

If the menu is a GBC only program, that kills SGB. Otherwise, a GB game can try to get SGB enhancements at any time. (A GBC game usually can't get enhancements, or gets confused if it does manage to get them)

To me, it'd be a perfect emulator instead, which would be the ultimate cart.

Is the real time clock really that necessary?

I've read that only Pokémon & Boktai (& a Rockman EXE game) use RTS.

Wait, is this a GB or GBA flashcart? Or DS?

Pokemon Gold/Silver had a real time clock. And that's why whenever you find a Pokemon Gold/Silver cartridge, the batteries are dead. But Zelda cartridges still hold a charge fine.

He means GB/GBC as GBA doesn't have MBC chips.

A simple solution would be to do like the 3-in-1s for DS Slot 1 do and flash a boot-able rom to non-volatile memory (like NAND or flash rom) and have it so if the device doesn't see an SD card in the device, it boots from said flash/nand/what have you and acts like a regular GB/GBC game.

This would allow you to use it with stuff like the SGB and N64 pokemon transfer pack.

Might be hard to fit that all in a small form factor though...