This is a senseless idea. Why would anyone want a BitUSD ETF?
Just use regular USD.
You would buy into a BitUSD ETF if you wanted exposure to the yield, which is better than a regular bank's interest rate. The Winklevoss twins want to allow anyone with a brokerage account to have access to Bitcoin - same reason applies here.
There could be different combinations of BitUSD and BTS, for example 90% or 50%, depending on how much of a steady return you want. The Bitcoin ETF cant give you that.
If this ETF were up and running and capturing yield for its holders, it would show how the BitShares system preserves value (transaction fees) efficiently and doesn't waste it like Bitcoin does.