Make a synthesizer that can accurately recreate all orcastra instruments convincingly. People will pay hundreds of dollars for it--maybe thousands.
Make a reliable and robust geometry kernel. People will pay thousands--or maybe millions for it.
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I'm not too familiar with synths, but aren't they good yet?
In the guitar world, you can get really good (for most people) modelers that emulate a good amp sound for like 100 bucks, and for 10 times that you can get _excellent_ sound.
My expectation would be that for similar prices you could get really good synths as well by now.
Synths that replicate real instruments are almost always sampled, which means they're just fancy recordings of real instruments. The problem with a recording is you are limited in how much you can adjust it; you cannot customize it as much as you can with algorithmically generated sounds.
I think this is simply because nobody knows an algorithm to create realistic piano sounds, etc. There are some good attempts, but they are paid and expensive. Hence my suggestion here.