I outlined the design I am contemplating in the blog and in the comments which ensued at Reddit which I copied into comments here.

Concisely, Stealth addresses for the high volume, real-time transactions and an optional Zerocash technology mixer denominated in the same token units, i.e. you can transfer you tokens into the mixer without any exchange risk, then transfer them back out to transact with the Stealth addresses. You could review my replies to @smooth, @JollyMort, and @jonas_h on Reddit for more details wherein I covered the issues about the trusted setup, slow speed of creating Zerocash transactions, etc..

Also another factor is that my Bitnet altcoin will not use proof-of-stake nor proof-of-work and it will burn the transaction fees (new blog is coming about this), so I claim this would make any sustained Sybil attack much more costly (and besides the Zerocash technology mixer is less susceptible to Sybil attack for the reasons I explained in the blog).