The one feature I'd like to see out of an ambience mixer app is a post-processor. If you gave me a nice digital reverb running through a compressor into a peak limiter, I think I'd be on cloud 9.
Then, to control the amplitude of the overlapping recordings, it would be wise to run it into a compressor. It's a little hard to explain how it works, so it's probably a better idea to link to a YouTube video[0] instead. The basic gist is that it brings sounds together, reducing the range between their loudest and quietest points (colloquially known as 'dynamic range').
Finally, a peak limiter is a more extreme version of a compressor that prevents a signal from becoming too loud and risking damage to your audio equipment. It's mostly a safety net, but it's a very necessary one when you start toying with dynamic range.
Hope this was able to help!
I'm just playing around with adding reverb on some of the clips (using Tone.js) and it doesn't sound all that dramatic to me. Is this really a feature that you think is noticeable and/or that you would want?