Research.

[stormbringer3]

When looking at Austria-Hungary and the Ottoman Empire, what are some of the better choices after trench warfare and infantry weapons?
Thanks for any opinions.

[lwarmonger]

Infantry warfare and command and control. These significantly improve the fighting abilities of your infantry, thus reducing your casualties and the burden on the German infantry. I also chose to research Artillery and Shell production when I had spare MPP... this enables them to go on the offensive.

[Xsillione]

And surprisingly I would say armor warfare, for 100 mpp you can have a tank, and if you put 100 more into it, you can have a tank almost at the same time as Germany or the Entente big ones (and later the second one, but that not really important), and for the low price of 200, they are insanely powerful units, so you can have three as the central powers, in theory at the same time and place, while the Entente will struggle to pull the same, as Russia will be on the other side of the conflict.