Well, if the Hungarians never come, expect unspeakably massive butterflies to occur, and not only in central Europe.
In OTL, Svätopluk, despite some of the animosity, actually struck a deal with the Hungarian tribal leaders and formed a military alliance with them against the Franks (but particularly against other Slavs in territories he planned to subdue and integrate into his empire). The alliance was alive for a few years, until Svätopluk's death and the inevitable balkanisation that came with it. While the usual fanciful legend about his three sons not listening to the advice given to them at his deathbed isn't completely distanced from the truth, the instability didn't just boil down to his trio of scions.
Svätopluk's empire at its height was far bigger than the original Great Moravia left over by his uncle Rastislav. The original was basically just Moravia and most of western Slovakia, but Svätopluk greatly expanded it, adding literally overnight territories comprising modern day eastern Slovakia, southern Poland/Gallicia, the entirety of the proto-Czech tribal lands and northwest and northern Hungary. He was close to retaking Pribina's and Koceľ's exile fiefdom on the shores of Balaton, but failed doing so because of a lack of time and Frankish backing for both Pribina and his son (Koceľ). Pribina and his kids being the original rulers of the Nitran principality (the Slovak half that gave birth to GM after the merger with House Mojmír-ruled Moravia), they would certainly have revanchist feelings of retaking at least a bit of their ancestral turf from Svätopluk's dead empire (they'd preferrably do so with Frankish help from OTL Austria and Carniola). And while Svätopluk also made buddies with the Franks, he started to piss them off in the final years of his reign (western expansion and all). What's worse - most of the newly conquered territories were still getting used to being ruled from Mikulčice (or wherever GM's true capital or seat of power was). Remember what happened to Alexander the Great ? Big Hellenistic Empire built in only a few years which quickly destabilized after his death and broke down into several successor states ? Well, picture the same thing happening once the Czech, Lusatian, proto-Polish, OTL East Slovak and OTL north Hungarian Slavic tribes learn of the duke's death...
If Great Moravia loses its rallying figurehead and then balkanizes due to the obvious potential for infighting and/or rebellions across the newly conquered territories (particularly Czechland/Bohemia proper, Lusatia and the OTL southern Polish lands), it would be rather similar to the OTL outcome. But, given the absence of Hungarian tribes, the campaigns of Svätopluk's later reign could turn out differently, and so could his reign. After it desintegrates, there is a very high probability that the Czech tribes, Pribina and his descendants and other local factions will ally with the Franks and dismantle what's left of the western half of the empire and its historical core. Without Hungarian interference in central Europe, those areas would be ripe for the taking. Basically, only the eastern, more recent additions, could probably escape both the Franks and the mightier western or southern Slavic polities.
A really tricky thing is to keep Moravians alive as a distinctly separate nationality - moreso than on a superficial OTL level. Moravia becoming an independent principality again (or later even kingdom) once Great Moravia collapses is a logical outcome, but one that is hard to keep alive if the Czech tribes unite in a similar timeframe as in OTL and get backing by the Franks (and later, early Germans).
In any case, Slovaks as we knew them will be butterflied away completely. I can see their OTL ancestors splitting into at least two separate nationalities, or becoming assimilated by their more powerful neighbours. Pribina's exiles around Balaton will be influenced greatly by south Slavs and east Franks, most of central and eastern Slovakia will probably come under whatever Polish/Lechic/Gallician/Volhynian tribes or polities that might create a powerful enough alt-Poland or some Rutheno-Carpathian Slavic kingdom. Dialects will increasingly grow apart and form into new languages, with only basic (if any) mutual intelligibility. Without a Kingdom of Hungary or Hungarian culture to influence them and their linguistics and contact with other local Slavic nationalities, even a united Carpathian-Pannonian Slav population within the borders of OTL Slovakia will not be analogous to OTL Slovaks, nor will it be as closely linked as them (on both a linguistic and cultural level).
Furthermore, as Marko has pointed out, GM's borders (especially at its height) are sketchy and more or less hypothetical.
Also, take a look at these posts:
https://www.alternatehistory.com/discussion/showpost.php?p=5957436&postcount=6
https://www.alternatehistory.com/discussion/showpost.php?p=5960881&postcount=30
https://www.alternatehistory.com/discussion/showpost.php?p=5962707&postcount=41
