All of this will probably end up with an hybrid system.
Something like the Euronext and its siblings.
Most of the time, this exchange works like the Nasdaq.
But, in case there is an important imbalance is selling vs buying pressure in the trading day, it goes in auction mode ("dynamic resa") , although no specialist is involved.
It also goes in auction mode + halt on more severe cases (profit warning during the trading day, or the 9/11 events, for instance).
There is no premarket.
So it looks like a NYSE and NASDAQ mix, although entirely electronic, which seems to provide, at least IMHO, a fairer market to every participants.
OHLC