Ho boy, here we go!
-Noteskins. In particular, most of the time Beats will be played with the default 'line up of targets, tap the column to activate it' set up, and so the 'arrow' motif borrowed from DDR won't make sense. (It's useful in that it differentiates columns by shape, however, and will be useful if an on-screen d-pad or set of d-pads is used, so I wouldn't remove it.)
Here are some example noteskins that should be offered with the game:
For circles: http://www.keybeatonline.com/forums/sho ... php?t=2330 - orbular in particular, but possibly cybular as well
For bars: Notebar+ Metalbar and IIDXMB Notebar and O2Bar++
For diamonds: http://www.keybeatonline.com/forums/sho ... php?t=2655 or something based on it
Other possibilities for shapes: +, X, circles with bars along their middle (think the way Technika notes look), circle rims with hollow centers, circle rims + bars through the middle, circle rim + +, circle rim + X... (just to be exhaustive!)
In addition, when mines come in the default ones ITG uses are quite similar to orbular, and so it's useful to have something distinct from any note. Examples:
http://img.photobucket.com/albums/v216/ ... ine4x2.png Spiky Mine
http://i2.photobucket.com/albums/y20/we ... e4x2-1.png Red X
http://i199.photobucket.com/albums/aa20 ... ine4x2.png Red Dot
Other things that would be good: Black bombs (possibly with skull and crossbones on them, ala Point Blank, or nuclear symbols or something else), sawblades, anything else obviously non-note like a rotating axe or burning flame
-Freely alterable notefield characteristics. I should be able to place the columns along the narrow end, the wide end or split the left half and right half to the top and bottom of the narrow ends OR the center and have the notes scroll out from the center/in toward the center. In addition, properties about the column proportions should be modifiable, either in game for more interesting ones or in a config file. Examples:
-The % of the side that the receptors take up
-How big of a gap there is between columns, or alternatively how much they scrunch together. When combined with a % number of the side that they take up, this means that they must grow or shrink in size! (Stepmania mod equivalent: flip +% for scrunching, flip -% for spreading, mini -% for larger receptors+notes, mini +% for smaller receptors+notes
-The aspect ratio of the receptors e.g. how flattened the notes look. Mostly useful for bar noteskins that won't alter much under distortion (Stepmania mod equivalent: Actually, I don't think there is one!)
-How big the touchable part of a receptor is relative to its graphic, possibly resulting in overlap, and if I hit two receptors at once it should trigger both (and sliding should trigger/untrigger them as expected as I move my finger around (No stepmania mod equivalent because it doesn't have a touch screen interface x3)
-The receptor's 'vertical' position, how far from the edge they are or how far into it they recede (Stepmania equivalent: reverse +% for further from the edge, reverse -% for nearer to the edge)
-How far up from the receptors and along their lanes the receptors can still be touched and have it trigger. Should by default extend all the way imo (again no equivalent)
-If it's easy to implement, how far into the screen or out of it the lanes tilt. (think guitar hero scroll and star wars plot text respectively) (Stepmania mod equivalent: Distant for star wars text, I think hallway for guitar hero scroll)
-How transparent the receptors are and how transparent the notes are. If it lags too much, then just have the receptors toggle able, on or off (Stepmania mod equivalent: dark % for receptors, stealth % for notes)
-How big of a gap there is between the middle two columns (for even numbers of columns) or how widely stretched the center column is (for odd numbers of columns) (Stepmania equivalent: The first one can be done using some calculated % of flip and inverse I think, second one requires sm-ssc and modifying the noteskin but there are 7-key noteskins that make the middle wider)
Other imo vital mods:
-Hidden and sudden with adjustable offsets. Sudden in particular, because rhythm game pros train themselves to hit notes a certain number of milliseconds after they appear on screen, and so you have this thing in IIDX called a 'green number' which modifies how much of the screen is covered by a gray wall according to what speed the song is running at so it exactly matches this millisecond count. Hidden is also useful, since it hides information that should have already processed.
-You've mentioned implementing a floating d-pad, but it should be possible to have up to two floating d-pads, so you can play PSPRevolution or dualshock style. And of course they'll need to be manually positionable on the screen, their apparent and receptor size adjustable, closeness/farness, etc etc... but should have sane defaults that you can reset to if you mess it up. And it should definitely be possible to slide your fingers from button to button without needing to release and repress, or you lose a lot of the advantage such a playing style would give in the first place.
Things that flash to the beat/respond to your input. If you play DDR you'll see that the lifebar cycles and the receptors flash at a speed corresponding to the bpm. It's a nice touch, and it really leaves its biggest mark in 'x-mod gimmick' files, where the screen seems to respond to the way the chart speeds up and down and even pauses or stutters. In addition, the receptors 'push in' if you trigger them when nothing's there, which is useful information especially for people who don't have complete muscle memory yet, and there's a very brief flash when you hit a note successfully - and this flash is coloured the same as the judgement. (You don't want it to go for much longer than the duration of the hit itself, else you'll get confused by overlapping flashing things.)
When you play games like IIDX or pop'n, the idea of a hit flash is extended further. A gradient is lit up that's flung upward from the key you hit whenever you strike or absently mess around. It looks cool, but care has to be taken so it doesn't obscure readability. In pop'n, it's taken a step further - the colour of the flash differs depending on what judgement you got. Looks nice, and provides a clear visual feedback when and where you stuff up, which is important for training yourself - I should know when I make a mistake, so I can remember what I did wrong and think about how to correct myself next time. The note responding in some way to being CBed would be good, too - make it fade to black or pulse or burn or something, it shouldn't disappear as though I hit it perfectly - that's what keybeatonline currently does, and it looks odd.
-When you add 5key mode, could you add 6key and 8key too? 6key might take a bit of scrunching but you could feasibly fit it against the long side if the narrow side is simply too narrow, and use a thin noteskin like a bar type so the screen doesn't get too cluttered. Alternatively, for both 6key and 8key, you could do the thing I mentioned earlier where the notefield is cut in half and arranged in the center facing outward/on the outside facing in. You'll have to multitask a bit, but it is possible to play like this - Audition's beat'em up mode works like this.
-Vs mode? Tap tap revenge does it with the 'receptors on two narrow edges, notes come from middle' technique so it's definitely possible. They could both play single charts, split up a couples or double chart or play half of a 6key chart each.
-Maybe a smart modifier could automatically DJ Max Portable-ify a chart?
EDIT: I was working on an algorithm when I realized the problem is non-trivial. To wit, the problem is analogous to DDR's 'can this be done without doublestepping' problem: can this be played without crossing the thumbs over? This requires keeping track of which thumb is doing which: 343 is fine if your left thumb is used to hit 3 (e.g. 41434341), but not if your right thumb is used (e.g. 23432). But there's a problem: After a jump (especially one that isn't on one side of the chart) or at the beginning of the chart, it is ambiguous as to which thumb would be used, so you have to either make an assumption or keep both in mind and resolve it retroactively when you come to a problem. I still think it could be made, though - on a high level, it should be fed a certain 'threshold' and then rework all patterns that force you to use the same thumb on notes closer than threshold milliseconds, excluding those into and out of jump/chords. The pattern can be fixed by identifying the first note that forces you to double-thumb and shifting it to a column on the opposite side, and repeating for every note thereafter.
(And then one has to consider minijacks - if receptors are big enough to share, they can be hit with alternating thumbs, but if they're not then they're just as bad if not worse than other double-thumbing patterns.)
(Final note: If sliding is easy to time by a user (How would that work? Would taps activate if you hit anywhere on the receptor, but sliding only activate when it passes over a sweet spot in the center, so it can be accurately timed?) then a dj max portable -ifying mod is rendered unnecessary, as sliding can be used to double-thumb until you're in a position where you can alternate thumbs again, or slide with the other thumb or whatever.)