Well, frankly, I'm a big fan of "cross-platform", e.g. I'm pretty much only using windows for gaming but given the fact Crysis simply isn't developed cross-platform from the ground up, there is nothing that could be done by a mod team alone other than to implement a compatible own engine (and really, with your current man power this should take no less than approx 10+ years), so simply forget that. Also I had to laugh about the suggestion to port to Unigine. You guys often complain about performance, so you should take into account that especially Unigines LOD for terrain and/or water rendering is WAY inferior to the technology used in Crysis. Just imagine the impact of that when it comes to huge terrains, e.g. visible for planes.
And seriously, don't turn this thread into a Linux > MacOS > Unix > Windows discussion.