Low Hanging Fruit and Failure to Pluck

 You can make musical tones by drumming or brushing your cheek. You can adjust the sound very dynamically by shaping your lips and jaw. The only reason this isn't a traditional instrument is that it's too quiet. Doesn't exactly make itself heard as part of an orchestra. 

 Why did the culture stagnate after finding the electric guitar? Paste a microphone to your face and use electronic amplification. Electric mouth...thingy. How many other unusual and interesting sounds, that happen to be too quiet for traditional music, are out there in the world? Thousands? Millions? Like, we already had guitars. [Guitars, but distorted] is hardly a huge leap. 

 Stagnation occurs when the culture loses interest in exploration. You can't really run out of frontiers. You can only run out of the most childishly simplistic, in-your-face-obvious frontiers. 

 I was thinking about a 3D piano or harp. Rather than 1D strings, imagine each string was a pair, forming a grid. Long + short, long + medium, long + long, then medium + short and so on. Either stack grids on top of each other or go full network, either way forming a cube. This instrument would be completely infeasible to make mechanically, but it's almost trivial to make virtually. The network version wouldn't even be physically playable; you wouldn't be able to reach the inner nodes to pluck them.
 Why did the culture stagnate after the theramin? There's millions of possible virtual instruments. Guaranteed at least a dozen of them are as beautiful as a hang drum. 

 Going outside the box is as simple as genuinely wanting to, then not quitting until you get there. 

 Discovery fails when demand for discovery fails. Exploration stops when the culture gets old. Senescent, and stuck in its ways.
 Far too interested at winning in the last game, regardless of how stale that game is. Doesn't care about the external world, only about its own navel. Lies about it too. You could imagine a culture that said, "Yeah, that's enough," and stopped exploring because it was satisfied. This culture has to forcibly suppress exploration, to avoid embarrassing its fake exploration.

 I don't like guitars. Massively overused. Pianos are even worse.