i'm pretty sure most 13 year olds like myself and older will problably have a DS by the time they make a game like that, maybe, half a year. and for the multiplayer, ds is supposed actually be able to work wireless multiplayer if someone else doesn't have a cartridge (or card), kinda like single game pak mode on the gba, metroid hunters is one of the games utilizing this feature!
I also have an idea of putting the main gameplay screen on the bottom with the sub screen start menu on top, that way, instead of having to tranfer the arrow, or hookshot like item first person view to the bottom screen, u could automatically do it on the same screen as the gamplay, like the original orcarina of time. and since the bottom is touch screen, instead of using buttons to acess orginally c buttons, u could just use your thumb.