Hello and welcome to the forum.
A rythm action game for Ios would be awesome (and I'm not just saying that given the amount of time I've spent on rythm rage), so I definitely like the sound of this, especially if it encoorporates the ability to use the Iphone gyro to make gestures as well as touching the screen, physical movement in a rythm action game would certainly be a new mechanic, particularly since despite Smackme and Zany touch experimenting a little with the formula neither really took it too far.
As regards irritations or access niggles with Aps though, it depends upon how things work. The first and most major question is what roll is Voiceover playing in the game? Since concerns of ap design worries will change according to whether Voiceover is used for things like starting a new game, checking your score, choosing game options etc.
If voiceover is! to be used then most requirements are fairly logical, ie, that vo speaks all elements correctly and can navigate to them easily, (either by simple left right flicks or by directly running a finger across the screen), none of those irritating partial image controls with squiffy lables, buttons with indefinite position or uncertain properties, and none of those ultra fast popups that appear and vanish before vo has a chance to read them. This also goes for any in ap purchices if the game has them, since there's nothing quite as annoying as a game which it's impossible to buy or donate to (I can actually think of one game, which is very! accessible with vo, and completely impossible to actually pay for due to bad lables in it's in ap upgrades store).
The major thing about using Vo however is making sure that voiceover behaves itself when the arcade game portion actually starts, since it can be a little irritating, and rather confusing to start an arcade game and have to turn vo off just after you've hit the "start game button" especially if say for example you press the home button an extra time or take yourself out of the running ap and back onto the home screen, leaving the arcade game running.
If you want an example of an arcade audio game which extensively uses the phone to it's advantage, uses vo for it's controls and reading of in game text but stops vo every time you need to get into the action, check out Audio defense zombie arena from somethinelse.
On the other hand, if your planning self voicing menus and for the player to turn vo off when the ap is run, that's not necessarily a problem but has it's own requirements.
The first of these is an efficient sound interface with speech interupt that can get to the controls. There is nothing so annoying as having to listen to a game say "Press with one finger to move through the options and then press with two fingers to select one" each and every time you run the game without interuption. Obviously you need that information the first time you run the game but not every! time.
Similarly, hearing "Start game, starts a new game" with no interupt can get rather irritating.
Of self voiced menus, there are two main types I've seen. those that let you scroll through the options with a gesture, effectively using a system similar to Voiceover's left/right flicks, and those that work by using the corners of the screen as buttons, eg "Touch the top left corner of the screen to start a new game, the top right to replay your score, the bottom left to change options"
I personally prefer the scrolling menus method myself,though that could be a personal preference thing. This is because by using the corners as buttons method you are limited to only a few choices, effectively four corners that can be found easily within the game menu, thus you tend to have to hear the menu explanation more frequently since it's not usually possible to remember specifically what each button does, especially for functions you only need intermitantly.
Also, the onscreen buttons method has the problem that if your not sure of the menu choices or options, you need to replay all of them each time, which can get complex especially if you have a number of sub menus, where as menus that have more of a single flick and scroll method can be gone through much more quickly to find the option you want.
One other thing to bare in mind if the game is using self voiced menus, is how vo intigrates with options like viewing game center, in ap upgrades etc. Turning vo on isn't a problem, but certain gestures due to screen sensativity can be somewhat more of a pest without vo, for example one game I can think of needs vo turned off to play, but then instructs the player to slide a finger up the screen to access game center and turn vo back on. Often I've found myself exiting the screen, or turning on the game center then toggling back to the original ap window, or even turning on game center and hitting something I don't want because my slide went too far.
Of course, if it was just a self voiced option that said "click here for game center and turn vo back on" that wouldn't be a problem,
A blind legend does this with it's in ap upgrades.
but extra gestures without vo, particularly when accessing other parts of the Iphone can be somewhat urcsome.
Hope some of this information makes sence and is of use, and I look forward to the game.
Feel free to let us know how the game is going, and when you have some demos or public info we'd be glad to post some news, and of course add the game to the database when it's ready.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)