Hey! I was wondering the same thing, but couldn't find much, other than a gfa packer that didn't run on my pc for some reason, switch toolbox, and some videos showcasing modded in levels. I've also started making a guide with all my findings and...
I played Yoshi's Woolly World with a friend once and had a great time with it (only 2 players total though), other than that I would recommend NSMBU or Super Mario 3D World because they have great co-op. NSMBU also has a huge mod called Cloudy...
Eventhough the New 3DS XL is more powerful, I still feel like the DS Lite was a more polished system. It's a real shame that it never got an XL variant keeping the GBA slot. You'd have to go on AliExpress and buy an ML shell to give a DS phat the unofficial "DS Lite" treatment, and that's the best we'll ever get I'm afraid.
@Jayro, I don't see whats so special about the DS ML, its just a DS lite in a phat shell. At least the phat model had louder speakers, whereas the lite has a much better screen.