Adding Bulgaria is impossible, since the campaign map cannot be changed.
I have, again, modded my game to make poison gas linger for longer, and it has not caused any crashes so far, though the AI seems completely incapable of avoiding it. I should release my changes as a submod eventually.
Changes made so far:
More machine guns and artillery at the start of the campaign.
Research times reduced.
Warsaw given to Russia.
Posion gas stays much longer.
Adding trenches actually dug into the map would be a huge bonus. One way to do it might be to actually make them part of the map, and have certain "trench maps" activate if 2 armies engage after standing close to each other for more than 1 turn, or for haivng 1 army dug in if they have been static for more than 1 turn.