Well there were several point in time when that could have happened.
The earliest is to have Borna defeat and kill Ljudevit at fords of Kupa(819.) ending the rebellion. For his troubles he could have recieved lower pannonia to his rule. After all Borna went from "dux Guduscorum" (tribe living in todays Lika) to "dux Dalmatiae" to "dux Dalmatiae et Libburiae" in just sever years due to being a good Frankish vassal. So it is not beyond beliefe that following his success he could be granted lower pannonia. That would make Croatia by far the most powerfull south slav state for some time to come. It would not be beyond immagination that one of Bornas successors would seek to add the small slav polities dominated by Byzantium to the south of his realms as well as extending its influence to the Transdanubian slavs.
The second point in time is during the rule of King Tomislav(914-928). Along with Simeon of the Bulgars he was on of the people at the time you wanted to be in good relations with. Even though historical records most probably exafgerate the Croatian military might of that time (100 000 infantry, 60 000 cavalry, 80 larger ships with 40 warriors and a 100 smaller ships with 20 warriors) Tomislav comanded a formidable force. Sometimes between 914. and 924. there was a war beteen Croats and the Hungarians. Croats won and integrated lower pannonia (it would later become known as Slavonia) into their realm. Hungarians came to the panonian plain in 896. and there was between 20 000 and 30 000 of them mostly male warriors. It is not beyond imagination that after defeating them Tomislav could have marched further north beyond Drava and crush them utterly with the blissing of the Pope and the approval of the entire christian europe. Local slavs (which were christianized by then) which outnumbered the Hungarians would IMO stage a revolt against their overlords at the sign of an approaching christian slav army from the south. Once more the annexation of Transdanubia like lower pannonia is not beyond question.
The third point in time is to butterfly the assassination of king Miroslav(949.) by ban Pribina and then have Croats tag team with Otton after the Hungarian defeat at Lechfeld to crush the Hungarians and set up slav principalities in the area.
The fourth point in time is during the reign of king Zvonimir (1075-1089). This one could be a bit different. Zvonimir was married to Ilona the Fair of Hungary, sister to Ladislaus and Geiza of Hungary. Zvonimir had a son Radovan which died young. Having him survive he could claim rule over Hungary through his mother especially since close to the end of his life Ladislaus had strained relations with his other two nephes Koloman and Almos(sons of his brother Geiza, which he overthrew). So it is not beyond imagination that Ladislaus could have named Radovan his successors. Probably civil war would follow with Radovan having the backing of the Pope, add to that the Cuman invasion of eastern Hungary and you could easly get a personal union between Croatia and Hungary with Croatia as a senior partner.
If we keep balance between Hungary and Croatia as it was IOTL and have Slavonia switch hands depending on the internal stability of each kindom as well as the strong rulers that could extend their influence, there is another way to turn Croatia into a power in Europe. Make Croatia turn to the sea.
Ever since Croats and other south slavs came to the shores of the Adriatic they started building ships and raiding as far as Crete. Also they settled the Gargano peninsula but without support from the other side of the Adriatic the comunities got assimilated.
So a PoD with Croatia developing as a naval power with still considerabble land possessions would be these.
The earliest could be 846. The great dodge Pietro Tradonico (ruled 836-864) decided that under his reign he would stop Slavic raiders. In 839. he launched two campaigns one against Croatians and one against Narentiens. Both ended with failure and the dodge had to make peace offers to the slavs. In 840. another campaign was launched against Narentiens which ended in disaster. In 846. Narentians now had their turn to attack and breached the lagoon and torched the city of Kaorle and took the inhabitants to be sold on slave markets. Have Narentians torch Venice instead and the butterflies are huge. Suddenly the Policeman of Adriatic is gone and the slavs are mostly free to do what they want. The settlement of Gargano is strengthned and maybe some other towns recieve significant slav populations since they take over the trade. It not beyond imagination to have Croatia develop as a naval power akin to Venice, but one that has very good relations to the papacy. The Arab incursions into southern Italy are also differently handled. With cities like Zadar and Šibenik which had very good ports and had great geostrategical positions becoming the leading cities of the Adriatic.
The second PoD is during the reign of prince Domagoj (ruled 864-878), at that time the fighting between the Croats (and Narentians) and Venice were probably the fierces and Venice suffered greatly. Domagoj was so loathed in Venice that they named him "
pessimus Sclavorum dux" (the worst duke of the Slavs). There are some idications that Venice was so thoroughly defeated that it had to sign a peace treaty under which they had to pay Croatian rulers the right to sail the Adriatic. It is believed that this tribute was paid unitl the dinastic stuggles of 999/1000 when Venetians invaded Croatian coastline. It is quite possible to push the Venetians so far into the defeat that they have to surrender their entire fleet to the Croats and though remaining a traiding city of some importance in northern adriatic never realy ascend to their regional power position. With their place taken by the Croatian state.
The third PoD could be during the 999/1000. dinastic strugles. King Stjepan Držislav had three sons Svetoslav, Krešimir and Gojslav. Svetoslav becomes the next king but his two brothers dethrone him with the help of Samuilo of Bulgaria. Svetoslav asked the Venetians for help which used the opportunty to take over much of Dalmatian coast. Svetoslav lost and he and his son fled to Venice where Svetoslavs son was married to dodges daughter. IOTL there was soon after a coup and the Orseolo family was exiled to Hungary. From there Svetolsavs son using the instability in Croatia later conquered Slavonia with the help of his Hungarian cousines. The descendant of Svetoslavs son would be later king Zvonimir. If we butterfly the Orseolo coup it is not beyond belief that Svetoslavs son could at later date retake Croatia just like he did in Slavonia, just this time with the help of Venice and Byzantium following the defeat of Samuilo. That way through the union of ruling families of Croatia and Venice a union of sorts could grow.
The fourth PoD could also be during the reign of king Zvonimir. Since he was a descendant of the dodges he could claim that Venice belongs to him. Following the successful war he fought alongside the Normans against Byzantium and Venice it is not imposibile that he could have decided to "liberate" his rightful possession. But then again you would once more need Radovan to survive or the entire episode with ladislaus and Koloman happens again.
Separate from these apove PoD's are two other.
The first takes places in the late 11th century following the succession crisis following the death of king Zvonimir. The PoD is the battle at Gvozd where the Croatian claimant Peter Svačić/Snačić was defeated and killed by Koloman of Hungary. Let us say the opposite happens. The Hungarians are thrown north of Drava and Peter needs to strengthen the kingdom before taking the fight to Venice which holds much of the coast. How things develop from there is anyones guess. What is clear is that verly likely there would be no medieval Bosnian state and it would be interesting to see how Croatia would use (if it would use) the weakeing of Doclea in the early 12th century. Could Croatia at this PoD become an Empire? Maybe but it would have to be really lucky (with a string of capable kings) and would have to play the game of alliances masterfully to reach a status of an Empire.
The seecond takes place at the turn of the 13th into 14th century when the dinasty of Arpad has failed. At that time the most powerfull fedual lord in Croatia (and I think the second most powerful in the entire joint kingdom of Hungary-Croatia) was Pavao Šubić (one of his famous descendants would be Nikola/Miklos Šubić Zrinski, the defender of Siget/Szigetvar against Suleiman the Magnificent). In 1272 he became Ban of Croatia. During the civil war which saw Charles I of Anjou come to power Charles relied heavily on Pavao. He even granted him the hereditary title of Ban (that is one step from becoming king) and gave him all the lands between mountains of Gvozd and river Neretva. In 1299. Pavao become Lord of Bosnia and starts to issue his own money. In 1311 he conquers Zadar from Venice and at that time rules from Drava to the sea including Croatia, Slavonia, Bosnia and most of Dalmatia. Following the siege he died. In 1300. he was crucial in Charles arrival to Hungary, basicaly he invided him to come. Without Pavao's support Charles could have never become king of Hungary. So let us make 1299. the PoD. At that point Pavao has pretty much everything. So lets assume he summons the Croatian diet and concludes that since the Arpads have died out the union signed between the Croatian diet and the Kings of Hungary in 1102 is no void, Croatia is once more an independent kingdom. The Croatian diet raises him to the honour of the king (he had some very distant murky relations with the last Croatian king Petar). The Papal legate crowns him the King of Croatia in the church of St. Peter and Moses in Solin as Pavao Držislav II of Croatia. Hungary is left in chaos for sime time allowing him to consolidate his position and allowing one of the powerful Hungarian magantes (maybe Ugrin Csak) to the same what he did. Where the history leads from there is murky but very probably to wars with Venice for the rest of the Dalmatian coast and quite probably either good relations beween the kingdoms of Serbia and Croatia or Croatias interference into the Serbian dinastic battles since Pavao's daughter was married to someone whose name currently escapes me but that man would be one of the grandfathers of Emeror Dušan of Serbia. With an expansionist Croatia we could very likely see the Kingdom of Croatia becoming what Serbian Empire was IOTL.
Woah this was wordy