I would like to suggest that in addition to a barrel jack and 6-pin PCI-e Mini-Fit Jr power connector, that the board also have connectors to accept 12V power from a 4-pin Molex, and also from a SATA-style power connector.
Heck, add in some screw terminals too.
Also, an on-board header to power a fan would be a nice touch. Software-controlled fanspeed would be even nicer.
There will be a temperature controlled Fan header.
As for power connectors: adding all kinds of connectors would increase board size disproportionately.
Host software will be cgminer, it will appear like an orignal avalon to it.
But since it will be opensource and the firmware will be upgradeable via USB it is possible to run your own protocol/host-application.
burnin please have in mind 2 powering solutions(if possible): first one for a 1-3 boards and the second one for 10+ boards(stacking). I remember a pic from ngzhang that had 10+ stacks of Lancelot units and a ton of cables and adaptors. I bet people would love powering a stack of boards with one single PSU.