Well, DS is 802.11 comptatable, so imagin that a web browser is released in DC Card format, well, you'll need a place to store those game demot you'll be downloading from Nintendo.net, or whatever...
But then again, they coul just throw a crap load of blank space on to that web card itself, thus completely making my previous statement pointless. And that would be the more likley outcome.
I do like the simultanious access idea though. Its like, "Hey, lets collect all theres cool upgrades in Zelda: Minish Cap, now you can transfer thom over to Four Swords DS. Also, I belive it is most likely that the DS version of Action Replay will simply be a GBA cart you leav in as you play your DS game. Also, they could use the GBA slot for periphrials. Like, put a rumble pak type thing in there, or a motion sensor for another Kirby Tilt & Tumble game.
I don't use GBA Video, but I'm just wondering if the GBA Videos will play in the DS, seeing as how they do not in the GameBoy Player.
Speaking of the N64 Expantion Pak. There dosent seem to be a problem porting standard N64 games, as demonstrated by SM64DS. But I wonder how well expantion pak required games woul run, like Perfect Dark.