I'm serious, I think I will do it on vacation. I'm an huge ouendan fan, played every game and I'm sad that there isn't more. I'm doing it for me, on the first place XD!
It looks simple, palib have manyyyyyy functions to help on every way, like zooming the circles, changing colors, checking the touch. I don't know why nobody did this yet. It's impossible to put some things like the videos. The slider thing isn't difficult I think. There is already a thing to check the collision of the stylus with a sprite, so the rolling ball isn't the problem. The problem would be make an "path" for it, but it's not extremely needed, I can make a simple guide o__O.
And about the videos, why not put an animated gif on place, like on ouendan? On ouendan the background is like simple gifs animated. I can do that. There is a background that accepts simple gifs
!
There are some things I need to think to resolve, like the time (calculated on ms on .osu and on ds it's calculated... well 16,6666 ms every frame o___o! BUT I WILL OVERCOME THIS!!!!!!!!!!!1111111111111 OMG)
And the Osu! staff are more than willing to help the port. Seriously, I asked one guy and he answered me (saying that this is a thoug thing to program o__O), saying me to search for help on Irc (about the osu files, that i couldn't understand perfectly), and there is a topic on the foruns saying that if you want to make a port, contact them.
But I never did a single homebrew, so there is a chance that every thing I said is worth nothing .___.
If someone want to help me, send an PM
! If someone is already doing Osu, i can help! And if someone know where to find custom osu source, I would be happy too, lol. DDR DS source works too (so I can look how they solved the time problem

)
Sorry for the poor english ._.