I agree with you that this outcome is almost impossible.
When I scetched the above map I had this line of thought.
In year 623 the Samo uprising took place among the Slavs in the west of the Avar Khaganate, joined by the Slavs west of the Khaganate border. Unlike IOTL White Croats join this alliance/coalition/state. Three years later when Avars suffer a setback at Constantinopole. Samo moves eastwards, White Croats take most of todays Slovakia and Transdanubia. Bolstered by these events Red Croats living further east along the Carpathian ridge join the alliance/coalition/state making Croats as the dominant group in the a/c/s.
Following that the Khagan dies (as in OTL) and Khaganate colpases into infighting. White Croats move their centre from what would one day become Cracow to the position what would one day be Zalavar, on the shores of Balaton lake. In addition White Croats penetrate Dalmatia, where they are joined by allready settled Slavs in ousting the Avar war parties.
In year 631/2. Dagobert led 3 armies against Samo in OTL and was defeated, as a result White Serbs (Sorbs) joined Samo's a/c/s. With the power of Samo's realm eclipsing the fallen Khaganate and rising to match the Franks Slavs start a second phase of aggressive migrations in TTL.
From years 631/2.-641. Samo's Slavs raid Thuringia while raiding gropus of Croats reach as far south as Crete, establishing settlements in Atica, Argolid and on Crete, in OTL. Duirng the last year of that decade Samo supports duke Radulf of Thuringia against Sigebert III.
The next year (642.) Croats mount a naval raid on the town of Sipont on the Gargano peninsula on the other side of the Adriatic. The town is sacked. Slav colonisation of the peninsula begins.
Around 650. leading Croats accept Christianity from Rome. From the foothold of Gargano Slavs raid into Italy, clashing fiercly with the Lombards with the benediction of the Pope.
Eight years later Samo dies and Porga leader of the White Croats (them being the most powerful faction of the alliance/coalition/state) takes his place. As Byzantium Empire is under siege by the Arabs and Fraks are divided among themselves Pope turns his intentions heavily on the growing Slav "empire".
Around year 670. the Porga's realms destroys the remrants of the Avars in the eastern parts of the carpathian basin as well as re-directing fleeing bulgarian hordes against their enemies. One horde each is sent to Bavaria and Po valley where both are eventually defeated but inflict great devastation on the areas. The third, largest, horde under Asuparh is sent to besige Constantinopole. After weeks of heavy fighting Porga arrives at Constatinopole and assault Asuparhs camp destroying the Bulgars south of Volga. The next decade sees a slow but steady spread of Porga's control over the Balkans as well as further penetration into Italy.
Porga dies fighting a new wave of nomadic warriors during the middle of the 8th decade of the 7th century and is succeeded by his nephew Lobel. By the end of the 7th century the new wave of nomadic warriors has been defeated and the state has been solidified. Most of Italy has been over run by Slavs which enmase convert to chrisianity, due to the instance that only christians may own land, but with a distinctive flavoure coming from paganistic traditions.
In the Mediteraniean Slav raiders clash with Arabs in brutal fighting relieving some preasure from the Byzantineans. The Eastern Emperors cedde the Balaks to the Ban of the Slavs in return for military assistance against the Arabs.
On the first day of spring of the year 700., on the day of Easter a new ruler is proclaimed for the Slav "empire". A gathering happenes in Salona with the arrival of the Pope, various germanic and "roman" leaders from Italy, Dalmatia and the Balkans as well as many slav chieftains that could attend. On that day Višeslav is crowned the Holy Roman Emperor by the Pope and Ban of the Slavs by the great Žrec.
So the map would show areas that swore aligence to the H.R.E. Višeslav. The official it would be known as the H.R.E. in the West, everbody would know it as the Croatian Empire, since they would provide the ruling familiy as well as forming the bulk of the population as well as the military.