Given Magpul PTS' desire, but apparent difficulty, to put out an airsoft version of their MASADA/Bushmaster ACR, maybe G&G could work something out with them to help things along.
Since they already have various M14 bases, it probably wouldn't be too hard for them to put out an out-of-the-box EBR and/or M39 EMR (the USMC variant all manufacturers seem to be ignoring).
If they want to do something totally new and different, they could enter the LMG arena with a KAC Stoner 96, which no one has done yet (and someone really should). A '63 variant could be derived from it for the Vietnam -era players.
A Russian PKM could be another interesting LMG (to offset all the M60s and M249s), for the Russian and Mooj -themed players (and possibly PMC as well).