There's plenty that can still be done with Flashcarts, like:
Audio Enhancer and Equalizer for in-game and OS Browser audio
-Bass Boost
-Hall, Stadium, Live, Classical, Rock, Jazz, etc...
-Balance
-3D Surround Synthesis
-Professional grade audio enhancement modes (like HDTV's have)
Screenshot Taker (in-game)
Instant Replay Maker (in-game)
Real-time in game audio record
Full AVI/MPEG4/DIVX/MOV support (for when we all have 16GB+ cards)
Built-in Save File Extraction
Real-time cheat enable
In-game brightness, contrast AND color adjust (what a magnificent hacker you'd have to be!)
Get rom info while actually still in the game.
Get Date/Time while still in the game overlaying screen and in menu
Get FPS, memory usage and CPU usage while still in the game, actually displaying overlaying the screen and in special menu.
I'm a game designer.... these features are valuable to DS game evaluation and just for extra enjoyment.