All the same challenges that Joshua Levine faced when he built Island. Read the book "Dark Pools."How will you pay the staff? insurances ? servers ? offices ? 1$ per contract or whatever the price is on futures will barely cover your servers and maybe a little bit of technology, but you still have to build robust software, regulatory cost etc... etc...
There is a real market force at work here: a generation of people who were brought up with laptops and phones. They know how to write code, and are not afraid to use their skill to gain an edge by writing algorithms to analyze market data.
Exchanges can dam the flow with non-display fees and other devices, but the demand is there. The innovation cannot be suppressed.
I even question why we are pricing in pennies. Even a novice programmer knows that a floating point number (say a C++ double) can hold 10 significant figures. Why can't I pay a price of 31.2374866492 per share of CSCO? This penny pricing reminds me of 8th's pricing that we saw not too long ago.
The truth of the matter is that exchanges have again fallen behind technology; perhaps more importantly, have fallen behind the Information Generation.