I'm not quite understanding what you mean by "open source" in this
context. Both big FPGA companies have DVK with FMC connectors and
usually the schematic of these boards is published (some third party
boards do too, as you need to address the parts from your FPGA...).
As you most probably do not want to produce an exact copy of these
boards by yourself, this should be enough to start development of your
FPGA and subsequently your own system, if required.
For availability of IP cores check the tool chains and IP generators of