My vote is for any non-western platform. There's already a 5/2 (technically 5/3 incl. Chinese Kilos) ratio of American/hostile nation platforms.

DW is mainly a sub/ASW game, but its ASUW engine isn't too bad and even a controllable PCFG would be interesting. What I'm getting at is, say you wanted to make a multiplayer US vs. Iran or US. vs China scenario you could play as a US frigate, a US helo, a US ASW aircraft, or one of two types of US submarine. On the other side, you can play as an Iranian Kilo or one of two similar types of Chinese Kilo.

As Russia is not as much a potential threat as China, I'd suggest either a Chinese Song class or Jangwei class. As others have mentionned, a Sovremenny would be nice, but it would unbalance multiplayer a bit.