You're right, HD collections are definitely awesome. Certain games will be good no matter the decade, I've been playing Ratchet and Clank recently and have been meaning to get Jak.
As for handheld titles, the few 3d ports that I've seen are great, Mario 64 DS plays like a dream with the circle pad. But if you mean collections of older games from previous handheld generations, the number of HD remakes I can think of is horribly low. There are the Pokemon games and... not much else.
My number one priority would be GBA games on the 3ds eShop. I mean really, you can play the Battle Network series on the Wii U, but not on a handheld?
And rawrrie, there has been a ps2 title on the 3ds. Not only that, but it was Metal Gear Solid 3, which imo is one of the best ps2 games of all time!