Are there any plans for an Audio Mixer feature like that of Flowblade?

Yes they are on the roadmap. For the next cycle there is going to be a refactoring of the timeline code to be able to implement advanced trimming tools and right along (or after) that the audio mixer feature should be implemented.

I am willing to participate in a bounty for this to happen sooner...

Cheers

+1 for the audio mixer

I'm also willing to provide funding to implement the following features:
* An actual Ripple Delete tool (not as limited as the current one) with the same functions as that of Lightworks, Premiere Pro and Avid would be an amazing time saver (select multiple "titles" or "colour clips" move them up and down to create gaps and then select the titles/colour clips at the same time snaping the video clips together while removing the empty spaces).
* Being able to reduce preview quality in a more flexible way without the need of creating proxies would be equally as useful for bigger projects.