The key to making a rhythm game is usually simplicity. Some games just use a touch screen for all input. Others are slightly more complicated by allowing you to use all buttons and features of the system you are using. But if you're looking for a rhythm game that is just as complex as, say, League of Legends, those don't exist yet and I wonder that if something like that would exist, that it would be any good.
Jam with the Band (for DS as well), is on the max of complexity as it exists for rhythm games. The easiest mode of the game just uses one button, which you have to press in time according to the triggers on the screen. Once you put the difficulty up to Master, you can use all buttons on your system to honor your favorite songs. It doesn't stop at that: you can play a song on all of the instruments that make up the song, or you can play the song together with all your friends playing a different instrument, or you can just write your own songs in the game and play those as well. There is even an option to connect the DS to the Wii and have the TV output the song you're playing.
Retro/Grade has a challenge mode where all unlockables are spread on a big map with lots and lots of challenges. There are only ten base levels in the main campaign which are recycled here, but have a modifier attached: things like 'The level speed is 125%/80%', 'The camera is zoomed in', 'The level has been mirrored', and so on. Of course, there are also challenges to play levels perfectly. Just one miss results in a game over. But Retro/Grade has a twist: it lets you rewind levels. If you make a mistake, you can just go back (more specifically, forward) in time and address your mistakes. This, however, resets your multiplier, so for the perfect score you will still need to play perfectly.
I feel a special mention to the BIT.TRIP series here is also on it's place. All BIT.TRIP games and levels are set up in such a way that they can be perfected. It is, however, freaking hard. A 'normal' level in the BIT.TRIP series usually takes up to ten or fifteen minutes, and even if you make it that far, there's still a boss to take down. However, if you manage to do it, the color flashing word PERFECT appears next to your score. A reward for insanity.
That about sums it up! I really hope so see more great rhythm games in the future. What weird rhythm games can still show up? When will someone make an rhythm MMO, for example? Maybe someone can do cool rhythm stuff with the Occulus Rift or another VR device! Make the future of rhythm games- keep the beat alive!
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I really hope to get my hands on Elite Beat Agents sometime...