You still don't have any software (or hardware) synthesizers so you're just putting it all together. If you are composing music, you're gonna need MIDI, instruments, hardware synths, or software synths (usually VST; in Linux LADSPA or LV2)

Without that you have no idea which samples you're going to use. It may very well be the case you're gonna switch back and forth between composing with instruments and sequencing, while your sequencer doesn't speak MIDI. It is also best to do the mastering in the sequencer instead of in a WAV editor.

Why burden yourself with such pain? Why not simply use a device meant for this usage? You can run Fruity Loops on a Windows XP netbook/laptop (probably with USB and some other hardware), and it'll run much better than what we imagine.

Given the state of Linux audio development I won't hold my breath for this kind of application. There are not many people who will develop this open source.

While touch screens can be used to audio development there are much, much cooler and useful aspects. Like this one: Reactable.

However if you're determined I recommend to first take a look at the current applications and see if there are possibilities to port them over. It'll probably be easy to port them to Linux/ARM (Maemo), as they are already ported by the Debian project, but getting them to work well with the touchscreen interface with 800x480 in a usable manner will be a very difficult, daunting task because you're gonna give the whole interface a major face lift.