This sounds incredible I have absolutely no idea whats possible but could you potentially have a looping function? I guess primarily for live, so if you preset a tempo and loop length then it loops round recording each time so you can have a sort of additive progression.
So you might set 120bpm + 8 bars, then it will loop, so you could start with the kick, then move on to other sounds to build it up in real time. Its kind of the way I've started to use nanoloop, although with piano roll (or stave) you'd have more scope as you could in theory have a really long progression loop rather than being limited to the 16 nl grid notes.
I'm not sure how easy that would be to do with the d-pad as you might not be able to input notes fast enough (works well on nl for iphone because of the touch screen so its much faster), but if we managed to get an external controller, maybe similar to LSDJ keyboard then it would be easier? ..or even using a midi piano keyboard and an arduinoboy to control it?

I have no idea if that made sense, but yeah

I'm learning and finding things bit by bit myself.

Starting with this, reading up.
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Will the source be available for this once you're finished?
I've been trying to learn more about Game Boy programming but it's been hard. -_-

Wow, thanks everyone. I've got a lot to think about.

I'm drawing out interface ideas on 160x144 grids now. I think the screen size is what will limit me most as to how many bars can be on the screen at a time. Controls are another thing, I'll have to avoid clutter for the screen size, but don't want to have too many sub-menus.

I'll share a few things i'd wish to see on my project:
*Preset drum patterns. I hate writing drum scores
*Chord Symbols for easy arppegios. E.g. IImin7, V7, Imaj7. Would speed up the composing process.
*Save and export as midi. 
*Compatable with Gameboy printer? I guess it'd be reduntant (and too much effort) if you could just export it to PC and print it on A4.

I thought about having preset sounds that catergorise sounds with moods for newer composers, though sounds are subjective. Maybe words like 'fat', 'pure' and 'dirty' might work.

Until i have better knowledge of what's capable on a gameboy such as whether it can recognise a keyboard, plans of linking up a keyboard etc to a stock GB may seem difficult. If it's a matter of software recognising it, then maybe, it'd make life alot easier no doubt. My coding knowledge is limited however. Gameboy uses C#?
There's a Dr. at school who has made his own Instrument (Tim Opie. He's into Granular Synthesis) before and has made open source music programs that can help me with some of this stuff, though the project will be my own journey with help here and there.

This is all very good feedback. Keep it coming.

yeah but that's a lot of effort to go through for a commonly used/wanted sound, which is why i said it'd be nice if there was a triangle waveform to select from in the first place

Just draw a triangle in the waveform editor, and set the instrument type to manual so it doesn't autmatically advance through the other frames.

is this some wicked old version of LSDJ you're talking about? it's definitely a sine in every version i've used.

no pressure.

YES! PLEASE. YES! This would be the best thing ever. I can't stand the tracker system (though it's starting to grow on me), and the piano roll interface of FLStudio and LMMS and Sound Club make me compose stuff in a weird way.

Of course, it would be cool if you could create it with the option to switch between staff interface and piano roll interface, though, but at least having just the sheet music interface would be just toooo guuuud.

As for suggestions, it'd be cool to be able to control the order of each note in an arp-"chord", perhaps with a little number on the side of each note, and maybe control the speed with a number underneath the staff. Effects could be handled "horizontal tracker" style, with an effects bar running along the top of the staff.

QFE

if this was done well it would be the best music software ever.
EVER.

This sounds cool. Although, I'm not sure if I'd actually use it that much, since one of the things I hated about using Sibelius was how long it took to input notes compared to a tracker, especially without a MIDI keyboard or anything.

I'd still try it out, though.

It is a triangle waveform... However, If you want a sine, set your instrument to triangle, manual, and lower the cutoff by a lot (I think)

this sounds cool as well

Sheet music's actually surprisingly easy to learn. Probably easier than learning to manipulate all of the values in most trackers.

A "learn to read music" feature would be a great addition.

Really cool idea tho I wouldn't be able to use it since I don't read music