The number of potential customers is too limited, so the developer's focus has been on single player simulations. The masses seem to only want eye candy and fast/easy play.
GraviTeam's work on WW2 tank sims is the most current. http://simhq.com/forum/ubbthreads.php/forums/238/1/Graviteam_Sims.html
RO and RO2, but I have stayed a way from that game engine for my own reasons.
There are some indy mods of other game engines, but the length of development depends on the amount of man hours they have available. Hopefully others will tell you which game mods to check out.
I am now under a 5 year NDA with Nordic Games for the Panzer Elite game code, so I can only hobby code unless a serious team could be created to do a commercial product combining PEA, PESE and PEx.
TPA who TWI