After some private discussion on a related topic, I reminded myself of a discussion that I started back in September of 2017 regarding the Maximum Spread and Usage of the Cyrillic Alphabet. Specifically;
The first half of my response to Kammada's suggestion was specifically what I reminded myself of. Whilst the religious implications would be incredibly interesting to discuss as well (what with Witching's non-appointment, and seeing the Pope appoint someone else as Methodius' successor, or even Gorazd attempting to convince the Pope to appoint him [which Methodius did OTL without the Pope's permission, resulting in Witching challenging the appointment]; as well as seeing what happens with the remainder of Group of Seven, those being Clement, Naum, Angelarius and Sava/Laurence, and the not-counted-for-but-still-mentioned Constantine of Preslav), I'm also rather interested in discussing what the political implication to such successful resistance would be; assuming of course that it is as easy as what I had just quoted!
So, what if Great Moravia had managed to resist the Carolingians?
An early POD would be the Czech and Moravian lands cleaving to the heritage of SS Cyril and Methodius (which would have required resisting some serious pressure from the West) and managing to spread that influence to Poland so that they all remain in the Eastern Orthodox camp after the Great Schism. The Slavonic Mass pretty much guarantees the chancery and lay use of Cyrillic.
The POD does seem plausible, but it would take much to achieve it. Having Louis the German's expedition against Moravia in 864 fail may benefit Rastislav's position. Carloman succeeding in his revolt in 862, during which he allied Rastislav, may also benefit Great Moravia, though in what way I'm not sure. If that doesn't work out, having Carloman's and Charles the Fat's engagements in 868 fail as well would help. Eliminating Svatopluk, Rastislav's nephew, would also possibly work, for he ended up entering in negotiations with Carloman without Rastislav's knowledge, and accepted Carloman's lordship over his person and his realm, thus setting himself up as his successor; so killing him off either before he does that or having him not be warned over the conspiracy to kill him, leading to his assassination, would work out well.
If we assume Rastislav's rule stays stable enough and successful enough against the Franks, then he'd more than likely have his son (which we could assume he had, possibly counting Bořivoj and Gorazd, albeit unconfirmed) succeed him. Unlike with Svatopluk, we wouldn't see a conspiracy form against Methodius and his disciples, preventing the promotion of the Swabian priest Witching as Bishop of Nitra, and thus, after Methodius' death in 885, preventing the banishment of Methodius' disciples and those educated at the Great Moravian Academy founded by Cyril. Since there is no conspiracy influencing the Pope's views and opinions on what's going on with the Archdiocese in the region, as long as Methodius' successor continues to toe the line he did, then there shouldn't be any moves made religiously. Assuming things continue developing quite dandy, those educated at the Academy may head north towards Poland and help in TTL's Baptism of Poland, and since they'd be against the German clergy, they'd more than likely follow the example of the Moravians.
The side-effect of this means that TTL!Cyrillic develops in Moravia or Poland rather than Bulgaria. Though linguistically things weren't much different yet, you know how those butterflies work.
The first half of my response to Kammada's suggestion was specifically what I reminded myself of. Whilst the religious implications would be incredibly interesting to discuss as well (what with Witching's non-appointment, and seeing the Pope appoint someone else as Methodius' successor, or even Gorazd attempting to convince the Pope to appoint him [which Methodius did OTL without the Pope's permission, resulting in Witching challenging the appointment]; as well as seeing what happens with the remainder of Group of Seven, those being Clement, Naum, Angelarius and Sava/Laurence, and the not-counted-for-but-still-mentioned Constantine of Preslav), I'm also rather interested in discussing what the political implication to such successful resistance would be; assuming of course that it is as easy as what I had just quoted!
So, what if Great Moravia had managed to resist the Carolingians?