While there may be a few software synthesizers already, aRts was designed with one goal in mind: flexibility
So it wasn't designed to be a program where you get two (or ten) finished instruments that simply sound nice, and provide some parameters. Not even three effects and a mixer.
Everything - from the smallest building block up to complex structures (e.g. mixers with equalizers, effect banks, houndreds of buttons and parameters on the screen) - should be able to be buildable of the small modules aRts consists of. There shouldn't be something that couldn't be recombined, reconfigured, replugged, rerouted or redesigned. A virtual studio where everything can be put you want it to be.
Of course there will be instrument libraries, effect libraries, and collections of mixers, complex filters, midi processing units, etc. that come with aRts. But these are not inside the code, all of it will be written in aRts itself, so that everything is accessible via Arts Builder. This is still some way to go though - but we're getting there.
Sure enough, new synthesis modules can also easily be written and integrated in the aRts system.