Honestly, I think the best case scenario is Czechoslovakia winding up like a richer commie era Poland (1970s/1980s), then having a Velvet Revolution in the late 1980s. Czechoslovakia is likely a little too dogmatic for the "goulash communism" model of Hungary. Something like Poland, i.e. less censorship of imported foreign culture, slightly greater leniency, that's about as plausible as it gets. They would not let Czechoslovakia off the leash. Even Hungary was under permanent and
de facto illegal military occupation since 1956, for over thirty years. Czechoslovakia spent 23 years on house arrest in the compound of a nuclear-armed doomsday cult before it finally freed itself. Imagine the UK, France, or even post-war Germany being under foreign occupation that long and forced to pay for everything as "punishment", as "tribute" to the soviet empire.
They would still seek to undercut Dubcek and other semi-reformists, much like in OTL. The reason the USSR didn't back down from quashing liberalization is the simple and sobering reason that it wasn't in their power-hungry interests to allow East Block satellite states, all acquired via illegal coups in the 1940s, to liberalise or become more open societies. Dubcek's efforts were quelled just because he allowed and encouraged average citizens to complain about governmental, administrative and economic corruption, and not face grave repercussions for "daring to" speak up. 1960s people interviewed on camera on the streets were genuinely fearful to speak about this at first, but slowly, they started to, and even that was subdued, measured, self-censoring.
The USSR saw what was going on, that even more innocuous political taboos were being breached, and they flew off the handle near-immediately. This is why I also keep explaining to westerners for years, in vain, that the USSR had no genuine interest in genuine liberalization, and therefore timelines with an almost hippie-and-cumbaya liberal USSR are not rooted in actual historical trends. The USSR never gave up social, political and cultural repression, because it was their bread and butter of running an empire that refused to acknowledge it's still mentally an 18th/19th century empire, hiding behind wannabe-revolutionary and wannabe-messianic ideological rhetoric. The USSR existed to further the interests of its mostly ethnic Russian elites. Not to appease Czechoslovakia, Poland, Hungary or the equally illegally occupied Baltics.
What I find a much more underrated ATL possibility is not the Obvious Great Dates of History focus on 1968, but perhaps ocusing on a slightly different evolution of Czechoslovakia after 1989. Not necessarily on whether it stays together longer or not, that's not as interesting, actually, but focusing on Czechoslovakia "going the Baltic way" in terms of dealing with its communist past, particularly the various communist officials who enjoyed complete impunity and unaccountability under the regime, even in the 1980s. After communism fell and the Baltics regained their independence, an important thing that Estonia, Latvia and Lithuania did was investigate their communist era officials, and made various legal precautions to prevent the old bashaws from running for political office or spreading demagoguery in local media. The judicial system was also reformed to move away from soviet era malarkey to western-style courts and their transparency, etc. Czechoslovakia did much more sloppier work in this area during the early 1990s, when it had a golden opportunity to at least attempt this. In the post-communist years and the dissolution of Czechoslovakia, the judiciary stuff was somewhat better handled in Czechia, but was largelly stunted and shoved aside in 1990s Slovakia, and the consequences of that - including half-hearted reform attempts since the 1990s - are still causing major sociopolitical problems in Slovakia to date, to the very present day. As much as things have advanced and improved in central Europe in the over thirty years since 1989, dealing with the ugly legacies of the communist era in an earlier and more effective way would offer a lot more possibilities in what modern history ATLs you could create.
Ironically it's not 1968 buA more intriguing POD dealing with Czechoslovakia and the communists would be any POD in the mid-1940s and any POD between 1945 and 1948 (yes, I'm hinting at diminishing the soviet post-war influence and especially the anti-democratic coup that installed the commies into power). By 1968, it's a little too late, short of the USSR deciding to collapse already in the 1960s, just for the lulz or something.