first of all, Thank you for this detailed answer!
I'm aware of the thermal aspects in electronics in general, but the fact that FPGA-timing is temperature depending was useful!
In addition to thermal issues, I'm interested in EMC, so that's why I was asking about the stack-up and common groundplate. A huge copper plate will likely cause much more Trouble under external Radiation than just single ground lines. On the other side it's a bigger heat sink for distributing thermal gradients equal.
The amount of Connections (balls) is obvious, but unfortunately I didn't check for this in the datasheet... Thanks!
So by now I'm on research but still grateful for any additional comments or Inputs (links, datasheets, ...)