Funny thing is that Youjo Senki came out in January and my 2nd cousin had an idea like the premise of the original post for a fanfiction (a Hapsburg Empire was a discarded idea by the actual author because he realized the past has nothing to do with Tanya's adventures). Picking up the author's stillborn idea, he has history start diverging by 1500s. In the 1710s a Maria Theresa (who doesn't look much like ours, or have personality) is supposed to pick up, but is attacked by Prussia, Sweden, Bavaria, Spain, Poland, and France. The Hapsbrugs win. Eventually, the Hapsburgs start collecting counties by inheritance in the HRE after their win.
In Prussia a few generation before, the rightful king was overthrown by his nephew. The Austria Succession challenger Prussian King was a grand grandchild of the usurper. One younger brother, the middle brother, was a Catholic and was a Margrave. The other brother (the one the king liked) was a fellow protastant. The middle brother adopted a kid, who had 3 counties from both his father and his mother.
Maria Theresa ran away from her regents and at the age of 13 hijacked command of an Austrian army whose commander died. The army sacks 3 French cities and defeats 3 different French armies in 5 pitched battles and an ambush. Her subordinates point out she is a good tactician, but, have no concept of anything that takes 170 hours to execute, to which she replies that what she had THEM for. They tried unsuccessfully to relieve her from command three times on the account of lacking a certain reproductive organ, but three counts from Austria backed their Archduchess who wasn't even supposed to rule yet (regency council). Many times they fought unnecessary battles against larger foes and losing meant annihilation while victory only led to modest material gains. In other words she could win battles, but didn't know which ones to fight and her response was "ok, you guys have the larger army and do what you want in Italy, I'm taking 25% of the army and going to Paris."
Shedding her siege train and supply line, everyone packs 7 days of rations and LOTS of ammo. Nothing is heavier than the 3 6 pounder guns. They surprise a French army 35K big and the 12 K Austrians defeat them and run down almost every last man. They bag 3K prisoners including the Dauphin, the siege train, provisions, and their cavalry runs down almost every Frenchman. They march onto Paris and quickly run out of artillery ammo. Maria Theresa demands the surrender of Paris, which the Austrians get the middle finger. Realizing that they are in danger of starvation, the Austrians spike their guns and make a getaway, only to be blocked by a French army. Maria Theresa arranges a fake retreat, and the French are like the British at Cowpens and fall for it hook line and sinker, but the larger French army still forces the Austrians in disarray after the initial Austrian gains and starts running them down with cavalry until the night. The Austrians suffer 3K casualties, and inflict 10K over the battle, but the problem is that they are broken into 3 groups which have more than 1K and one small group. The large groups left the battlefield unable to communicate with each other and each of the three separately retreat to the Rhine. One makes it to Hannover. One makes it to Hapsburg Italy. One makes it to the coast in Savoy. The small group contains 32 Horsemen and Maria Theresa. They go into Oldenburg and are arrested by the Count of Oldenburg who is allied with Prussia.
Alternate Maria Theresa, you suck as a child general. While the battles in the fanfic are plausible, the moronic counts who blocked all attempts to relieve her of command are really dumb, but we do have dumb generals in real life so I can't say that's unrealistic.
Austria was ruled by a regency council so nothing changes in the short run, there is a frantic exchange in letters from Vienna to Oldenburg, but surprisingly the Count does not hand over his prisoners. A 2nd cousin of Maria Theresa (same relatedness as me and the fic author) hijacks the regency council at the news of her captures and becomes de facto Archduke. Hungary's nobles refuses the cousin and styles their "King" Maria Theresa, starting a cold war with neither side willing to go at each other's throats while both are fighting the anti-Hapsburg enemies. The Hapsbrugs win in Italy, Barvarian, and Mediterranean cost of France. They hold off their other enemies to the north and the Ottomans who... actually there is a bug in this timeline there is no reason the Turkmen should be allied to the French. Well they are but they keep making tactical stupid decisions like trying to storm forts.
Three years later Maria Theresa comes into Vienna after a 30 day's notice, escorted by the count. She is accompanied by the Catholic brother of the King of Prussia, his daughters, and his adopted son. The son is actually the great grandchild of the overthrown king of Prussia and by male preference primogeniture, the rightful heir. He has the evidence to prove it too. What's more, the 17 boy and the 14 year old Maria Theresa both found each other cute and wanted to... do stuff but as Catholics needed to wait until marriage for some of that stuff. The fake Archduke (male but more distantly related) steps aside saying he only needed to rule since it was an emergency situation. He wanted to be elevated from count to Margrave for keeping the Hapsburg hereditary domains safe while spending 1/3 as much on military as France did. He also proudly announces that there are funds left in the treasury. If they mobilized every non garrison unit to the offensive (more expensive than passively waiting), used supply lines instead of foraging, and stopped collecting taxes, they could last for 38 months.
You know what's coming. The Hapsburgs have a war chest, have tactical battlefield victories, the British allied with them, their Archduchess safe, and a betrothal to a Prussian count who is revealed to be the true heir of Prussia.
After the war, the Hapsburgs collect titles over years by marriages. Eventually, Charles VII declares the creation of a unitary state with the approval of his nobles. Because he is an anglophile, it is "The Empire" not "Riech." Also, he wanted to deemphasize the German part because Hapsburg Monarchy, unity within the Empire, the Divine right of Kings, and German nationalism cannot all go together. The four are incompatible, any one of them can’t go with the other three. And guess which three are most important?
It still is very German though. It's the principal language and all nobles speak German.
Alas, this is an alien space bats timeline. Nothing I mentioned yet was fantasy. But in fact, the battles do contain low fantasy elements. Mages exist, but most of their offensive spells aren't useful due to long incantations. They can fly, but until the 1900s can only take off after 15 minutes of concentration, during which they can't shoot. In the War of Austrian Succession the flight is slow and in a straight line; they also can't shoot straight and maintain a flight spell until improvments are made generations later. So they are used like dragoons, only shooting after landing. The battles in the fic are plausible, both using in universe rules and during the non magical parts (most of them) are fairly plausible.
The only question is why the bad child general wasn't removed.