The British had plans to make mustard gas bombs around 1919. They never happenned, but I could see extending the chemical weapon tech tree to include them.
I suppose you would start with gas cillinders, that are released on the battle field, then gas artillery shells, then improved gas artillery shells, then gas bombs, then maybe improved gas bombs.
What if in WW1 one side started dropping mustard gas bombs on enemy cities? Probaby would have made them world enemy number 1 for sure, but probably would have had some other significant impacts.
Anyway in games like this, a player playing against the AI usually gets way ahead of where they are supposed to be historically, and extending the tech tree a bit would probably make it interesting.
Maybe it would just make it weird, since the programmers would just have to guess at what the implications would have been. It's just a thought.