I'm gonna have to agree with you there GameCopierUser0666... I have a GBA FA Bridge right in front of me, as well as a F2A card plugged into it currently. If I really wanted to, I could flash that F2A card and bust out all the GB/GBC games that I wanted, but I don't have much of a use for it at all, due to no system that plays GB/GBC games currently. Sure, I have a couple original mono GB systems packed away (the really big ones that first came out), but my GBA SP and my GBA both became mostly defunct when the GBA SP came out. Someday I might want to go retro with those systems, but in the meantime, it really seems like too much of a hastle for too little reward.
The idea of a NES flashcart however... Well, I was doing some research the other day, and I came across a device for the NES that never really lasted, although it did come out, and it was fully working, from what I understand...
The device worked by having ALL of the NES chips in one regular NES cart size, but it had a slot in that where you could put in littler games (the size of a GB cart I'm supposing). The system was supposed to make it so the games could be sold cheaper, because you wouldn't need to buy new chips every time you bought a game, and instead you'd only have to buy the program on a little game.
The device was called "The Aladdin Game Enhancer". Some details of how this device worked are on a site --hxxp://www.nesplayer.com/features/aladdin/aladdin.htm -- change the xx to tt to see the site, =oP
Before I go any further with this idea, I'm guessing right off that even though the NES is defunct hardware at this point, NOA is going stronger than ever, and I suppose they'd have a big time huff over neoflash putting all 11 or so MMC's on one cartridge and then having us flash some flashable eeproms on the cart to play whatever game ever on it...
But if I'm wrong, then do it up neoteam!

I realize this would be quite a challenge, but you know the dollar signs are just waiting for a project like this to hit the retro consumer.
Whatever happens, good luck!
EDIT: Well, I was just reading a bit more about the Aladdin Deck Enhancer, and I'm not sure what it had in it for MMC mappers. The other information on that product I had read prior to reading that article was older and lead me to believe something different. Oh well. Still an interesting concept I guess.