I want to take this opportunity before I do what I mentioned about offering chapters for those who pay and support me, because I'd like to hear your opinion about the story. I started thinking about all the possible scenarios this story could reach.
My idea is that Germany loses the war, but at the same time ends up in a relatively strong position,managing to endure the worst of the Spanish Flu, with the pandemic hitting the Americans harder, along with the offensives. That could force the United States to negotiate something lighter, so they don't keep taking the blow of millions of deaths in France and Germany.
From there, I see two main possibilities.
One option is that Germany gives up Alsace-Lorraine and rejects the Treaty of Brest-Litovsk, with only limited war reparations and some military restrictions, but without losing much more. That would lead to a story focused on how Karl rises in regional politics and industry while both sides prepare for a second round of world wars.
The other option is that Germany avoids reparations but gives up Poznan and Danzig, with almost no military restrictions. That would put Karl in a warlord-type scenario, defending his homeland from the Poles and trying to take control of the Brest-Litovsk territories to protect them from either the communists or the Tsarist loyalists during the civil war. At the same time, he would be labeled a traitor by Germany, since he would effectively seize several eastern armies using his prestige as a war hero, while France and the United Kingdom try to enforce Versailles, facing the logistical problem of how to reach Poland while dealing with someone waging a full guerrilla war against them.
At that point, I also have the option that in 1916, when Hindenburg imposes the military dictatorship, Karl is expelled from the army. He would then take advantage of that moment to go fully industrial, creating something similar to the Freikorps by forming his own private security force to defend the Brest-Litovsk territories. Using that power, he could force the Poles to grant him autonomy and favorable contracts with the newly formed states, since Karl would have the largest private army at that time.
But I'm interested in hearing what you think, or any ideas you might have.
If you prefer a Kaiserreich-style scenario, with a political rise and a war against emerging communist powers, that is still a viable option.