I recently started playing the mod and I was fooling around with SP games. Especially loved the Australians.
So I tried to play Serbia (well, I am a Serb and we are rarely a faction in RTS games), and I realised that, although they have some unique gimmicks great for rushing, they have hardly any unique units. Is this a design decision, or the team couldn't think of any new units?
I have some suggestions:
Serbian Free Corps, or Freikorps in German. They were mostly used by Austrian army as mercenaries. They freed a part of Serbia from Ottomans during Koča's Uprising at the end of the XVIII century, and they held it for around five years. A lot of veterans from that war, including Karageorge, have participated in First Serbian Uprising. They could be a slightly tankier infantry than chetniks, with bonuses against other infantry.
Harambashas were leaders of hajduks, the rank was similar to Ataman or Hetman to Cossacks. They could be a military unit that gives some bonuses to hajduk units, HP or movement speed. Also, there should be limited to 5 or 10 units.
3. Vojvoda or Knyaz
Serbian terms for military commanders and chiefs. Vojvoda could be a general that boosts other cavarly, and Knyaz could replace Boyars (Serbs havent used Boyar as a title since the Middle Ages anyway).
4. Black Hand operative
(Yes, that's their official seal)
"Black Hand" or "Unification or Death" was a organization responsible for the murder of Serbian king Alexander Obrenovic and had ties with Mlada Bosna, that assasinated Archduke Franz Ferdinand. They could be Serbian unique spy, like Italians have.