This game could do with a good multiplayer mode. I have a couple of ideas for different multiplayer modes:
1. Side by side Co-op (Duet) - You and your friend would both choose songs from your own library and play through the campaign with both players' songs mixed into the game as levels. Obviously to reduce level flooding you would more than likely choose only a couple of your best folders, unless your aim is to have a gigantic marathon campaign. The co-op campaign would be saved per machine, so if one of you accidentally deletes it your partner can load his copy of the save for you to play with. In the menus and upgrades parts you would only see your own screen, and to choose a song one of you would select it, and your partner would have to "okay" it; this would prevent you from starting a game when your partner isn't ready. The difficulty is set as a team as well, and your ships are individually customized. In the game itself, both ships are placed on the field (which has been widened to fit two players and the enemies that accompany) and the game is played like normal, only there are more enemies to accomodate both players.
2. One versus One (Bellicoso) - You and your opponent decide on a song from either library for the backdrop, and you customize your ships like normal. You don't choose difficulty because there are no enemies in this mode, only your opponent. This mode plays out similarly to a boss fight, in that there are no enemies and that Inspiration will appear randomly throughout the arena. You only start out with one weapon segment of your ship intact, and you can only gain your other weapon segments by collecting the Inspiration that pops up. Damage in this mode is indicated by a health bar rather than the number of ship segments you currently have. The number of ship segments has no bearing on the health bar, they are only used for weapons. It is still possible to lose segments of your ship if they take continuous fire, however. The arena is split into two halves, with one player on either side like a game of pong.
3. Duel (unknown name) - You and your opponent take turns choosing songs for the other to play on, and then you play them like a normal game. The only difference is that you can see how your opponent is doing on the other side (but can't hear their music so it doesn't mess with yours). Special to this mode are certain "attacks" you can do to your opponent, similar to a Tetris duel mode. As you gain higher chains, you can send more enemies after your opponent. Among these attacks are pick-ups that randomly appear on the field when you're doing good (not getting hit, getting high chains, etc.). These can range from blocking out parts of your opponent's arena with damaging tiles to reducing them to only one ship segment to making all of their enemies into minibosses (like the birds and the ships that explode on death, etc.). These power-up weapons only last for a few seconds but if you play them right that's all you'll need.
So Steam Community, do you have any multiplayer ideas for Symphony? Perhaps you like mine, but something just doesn't sit right? Tell me what you'd change, what you'd keep, what you'd add; and post your own ideas too, if you'd like.