Not exactly sure what the genre is called. but I'm looking for a game like Nanostray, While Contra 4 and Metal Slug 7 are excellent, I'm looking for somethign thats constatly scrolling, sending waves of enemies at you, spamming bullets, you get the idea =p
Space Invaders Extreme. Or there are a few homebrew ones, such as Touhou DS. Surprisingly very good for a homebrew. Seems to be very heavily based on that game on the Neogeo though (similar to 1942). Forgot it's name.
There is also a recently good one called Little Red Riding Hood Zombie BBQ. On this site it's Zombie BBQ when searching for it I believe. Well try that game it's scrolling shooter with massive zombies swarming at you.
may not be EXACTLY what your looking for, but, how bout installing nesDS. Then you can play all the shooters from the NES. The entire gradius series were wonderful. The contra series were good. gunsmoke was a great shooter. 1942 and 1943 were great. I'm sure there are a zillion more i have not mentioned.