Yeah, I can see the logic behind this, though I don't know if all the brothers would unite against Charles II without him taking at least one of them to his side (and Charles was presumably most militarily competent among sons of Charlemagne, as he is credited with victories over the Slavs Charlemagne made. Well, if Charles doesn't lose Thuringia that makes Bernhard inheriting Hugh and Drogo's realm problematic, as he has no access to them.
Just because Charles/Karl II is Militarily Competent doesn't mean he would be able to be everywhere at once. He'd likely have to pick which was the larger threat and would decide that Louis' Aquitaine and Lothar's Burgundy would be the more powerful threat due to its size and proximity to the heart of his Kingdom Economically and Population-wise. Pepin is the only son most likely to remain Neutral since he'd be busy dealing with the Lombards himself. The only brother that I could maybe see allying with Charles/Karl II is Drogo due to how weak Alamannia is compared to Bavaria and Saxony and how he knows he's toast either way if Charles/Karl II loses OTL East Francia. Then there's also the fact that the King wasn't the end all, be all of campaigns and the like at the time. If the nobles of Francia decide the war isn't in their interests because of their own greed/ambition/vikings, that's it, the War ends in a victory for the Brothers and they can carve the map however they like. A King at this time was only as powerful as his nobles allowed and if they decided it wasn't in their interests to keep fighting, then the war would stop.
Well, there is little to subdue on Balkans besides Bulgaria and Franks won't give up their influence in Croatia. Serbia was still a tribal shebang, though conquering it might be harder than conquering semi-centralized state.
Maybe Serbia could become something like a vassal state of the Empire that Constantinople focuses on integrating later on? It's kinda like what I plan for Russia ITTL with the South being filled with Byzantine Vassal states ruled by a Greek Orthodox Elite and the North being filled with German vassal states ruled by a German Catholic (at least, initially) Elite, leading to the two Russias to diverge linguistically, writing-wise, and religiously.
I think he would stay. Emperor of Byzantium is christian emperor and at least on paper of the same faith as Pope. Popes stayed in Rome when the situation was far worse for them, for example when Arian heretic Goths took over Rome.
Interesting. Do you think the Pope might raise hell against the Byzantines? Its clear the Papacy at this point views itself as a separate entity from the Eastern Orthodox Church/Eastern Chalcedonian Church (I personally think the last time tensions between the two churches could've been healed was during Justinian I's time) so I imagine they'd be a thorn in the side of the Emperor every now and again.
Well, than all of them are fit to lead a campaign. I'd give different question - what makes you think Omurtag wouldn't be butterflied? I do not see any reason for Krum not ascending to Bulgarian throne.
That is a fair argument, however I do think that some things about the regency might be butterflied.
Well, the conquest is kinda problematic because Franks were already stretched out and they already extracted tribute from Moravians (in 822 Mojmir I paid tribute to Louis the Pious, he'd do so for Charlemagne as well) and western Poland was an enigma at those times, though Einhard claims that Charles the Great subjugated all peoples until Vistula (well, some historians for that reason make equation between prince Lech who was slain by Charles the Younger's forces and legendary creator of Polish state of that name, but there is no definite proof for that), though we should notice Franks didn't exactly know how Danube flows and had no reliable maps.
Yeah, that makes sense in some aspects, though I do see him snatching up some borderlands and turning them into Marches.
For the Byzantines, it still won't be easy. Bulgaria is at its early zenith and will be able to resist attacks with relative ease much like it did OTL, unless you want the butterflies to make Constantine VI in the competent ruler that he historically wasn't; he may well still employ Nikephoros and benefit from his financial acumen, though, I suppose.
The historical trend though is against the Byzantines, so you'll need a fair string of good emperors to actually hold on against the various ascendant threats while having to care more than OTL about Italy.
It seems that Constantine VI didn't trust Nikephoros a whole lot as he had him blinded, though that could be more due to Irene influencing him to make that decision rather than Constantine himself. Could a competent General who is a friend of C6 lead military conquests in the Balkans as Rotrude uses her husband to pass reforms to help rebuild the Empire to prepare it for their sons?
As for a string of good emperors, I plan for an era of the 5 Tri-Emperors where there's 3 Emperors at any given point for 88 Years (Leo V, Constantine VII, & Justinian III ---> Anastasios III (Son of Leo V), Constans III (Son of Constantine VII), & Justin III (Son of Justinian III) ---> Maurice II (Son of Anastasios III), Constantine VIII (Son of Constans III), & Leo VI (Son of Justin III) ---> Maurice III (Son of Maurice II), Leo VII (Son of Constantine VIII), & Constantine IX (Son of Leo VI) [He marries Leo VII's sister Theodosia] ---> John I (Son of Maurice III (died without male issue, only one daughter, Constantia, was born)), Leo VIII (Son of Leo VII; Died w/o Issue), & Constantine X (Son of Constantine IX and Theodosia; Married Constantia of the Leonid line to re-unite the three Isaurian lines with the birth of Constantine XI)) with an agreement for one line to focus on one place in particular and not try to be more dominant than the other line for the sake of the Empire's stability (The Leonid Line of Leo V, Anastasios III, Maurice II, Maurice III, and John I focuses on the Middle East/Asian Portion of the Empire (being the Senior most line); The Constantine line of Constantine VII, Constans III, Constantine VIII, Leo VII, and Leo VIII focuses on the Italian portion of the Empire (Being the 2nd line); and the Justinian line of Justinian III, Justin III, Leo VI, Constantine IX, and Constantine X focuses on the Balkans until Constantine XI unites the three lines by being descended from all of them). Think that could be a good length of time for the Empire to keep things together and defeat all the threats targeting it?
As for the Pope, he'll stay in Rome; without Irene, whoever sits in Constantinople still is the Emperor, and as the Iconoclast movement dies down (like it did OTL) tensions will gradually reduce, especially if Byzantine arms are back to "protect" the Pope. Do note that doing so will require a clash with the still fairly robust Lombard duchies of South Italy.
Interesting. How long do you think it'd take to conquer the Duchies and the Papacy and how long do you think it'd take to subdue it? I imagine the first one is relatively quick but the second one is a long generational process.
Charlemagne seemed to be interested in the people of Slovenia and Croatia as a potential further avenue of expansion, though that was also because of the conflict with Byzantium his coronation had brought. He will certainly keep on warring until the very end, though, because he was one such man; though his many sons surviving likely mean earlier trouble and lots of hard to predict politicking (which means you are free to deal with it however you please). I would certainly expect some eager son to rebel against their mighty dad, and get ousted or killed for their trouble though.
Any ideas what further lands he might try to conquer without having a conflict with Constantinople by being Holy Roman Emperor (as that title doesn't exist ITTL)? I imagine Bohemia-Moravia and Brandenburg/Pomerania/Denmark/maybe Western Poland will be a place he has a lot of focus on going forward).
On his sons, I imagine it'd be the youngest son(s) rebelling as Berengar was planned to have Barcelona before his OTL death which is really nothing compared to the Frankish heartlands of Charles/Karl II "
the Younger", Italy of Carloman/Pepin "
of Italy", Aquitaine of Louis/Lois I "
the Pious", Burgundy of Lothar "
the Simple", Bavaria of Hugh "
the Bavarian", and Saxony of Bernhard "
the Strong" with Drogo weighing his options as Alamannia is pretty much a nothing territory but it'd certainly be better than no territory at all which is how a failed Rebellion against Charlemagne would end up for both the youngest sons if they survive the war. I also do plan for the Mercian double marriage to happen ittl with Ælfflæd, daughter of King Offa, marrying Charles/Karl II "
the Younger" and Bertha, Daughter of Charlemagne, marrying Ecgfrith, Son of King of Offa and future King of Mercia for 141 Days in 796 (he lives longer ittl).
On the Balkans itself, I don't plan for the Empire proper to go outside the South Danube with it stopping at the modern border Bosnia shares with Montenegro and Sothern Serbia (hopefully you get what I mean what I'm talking about here.
If you want that, that's entirely fair just that achieving it won't be easy and as I said the better way would be have Slavs destroy themselves from within and Germans come and pick up the pieces.