For the purposes of this work, we assume that a Slavic state roughly corresponding to the Duchy of Pannonia in the 8th and 9th centuries exists to the west of the Danube river, but east and north of modern Slovenia and Croatia respectively. I call the language spoken in this state Pannonian (jezik panoski), although the exact pronunciation is subject to change of course. The major POD is that the Hungarians, for whatever reason, are defeated at the Battle of Pressburg, and are pushed out of western Pannonia by Braslav (a major historical figure to the Pannonians). Therefore, an independent Slavic kingdom survives for several centuries until its annexation by Austria via Bavarian Nobles. However, the majority of Pannonia speaks a Slavic language until the present day, but there is Germanic influence comparable to that on Czech.
Below, I start out detailing the major sound changes between Proto-Slavic and Pannonian. Once that is done, I will then add a phonological chart and begin working out what the grammar might be like due to the sound changes I've included.
(Earlier OP spoiled below)
Merger of /ɨ/ and /i/: One of the earliest differences between the Slavic languages is their treatment of /ɨ/. South Slavic and West Slavic both fronted this phoneme to /i/, however, West Slavic left a trace of palatalization before the fronting. Hungarian also had /ɯ/, which was later fronted and merged with /i/. There is little doubt in my mind that Pannonian Slavic too, will front /ɨ/ and completely merge it with /i/ everywhere. Whether or not a distinction remains between former /i/ and /ɨ/ in the palatalization of the consonants will be treated later.
Chain shift: A common occurance in many Prekmurje Slovene dialects, as well as some Kajkavian dialects of Croatia, is a general back vowel shift of /a/ > /ɑ/ > /o/ > /u/ > /y/. Perhaps due to an Avar or Hungarian adstrate, many dialects of Pannonian will also participate in this sound shift. Most Pannonian dialects will feature at least a shift of /u/ > /y/. Northern dialects will unround this /y/ to /i/. Central and Eastern dialects, including the prestigious dialects of Lake Balaton, will also innovate phonemic /ø/. Southern dialects may not have fronted /u/ at all, or perhaps only fronted them to /ʉ/.
Fate of the yers (Havlik's Law): In accented, or “strong” syllables, the yers lower to /e/ and /o/ respectively. In unaccented syllables, they drop, perhaps leaving a trace of palatalization. All of this occurs between the 9th and 10th centuries.
Outcome of /ǫ/: According to Richards, there are a lot of borrowings of Slavic words into Hungarian point to the phonetic value of Common Slavic /ǫ/ as /ų/. Like Serbocroatian, the value of Pannonian /ǫ/ is /ų/, and this later denasalizes to /u/.
Outcome of /ę/: In most Croatian dialects, this vowel merges straight with /e/. In Czech, this led to a 2-way split between /a/ and /e/ depending on whether it was short or long. In Slovak, the quality of this vowel is preserved as is, with no change — /æ/. Although I have no evidence to suggest for it, I believe Pannonian dialects will preserve /æ/, unless I can find some dialects in central Slovakia that run the contrary, or loans in Hungarian that suggest otherwise. My romance conlang Pelsodian, too, has [æ] for its /a/, and so does its Romance neighbors to the west (barring the Dalmatian language). Therefore, I have an incentive to preserve /æ/, though I am still open to change.
Outcome of *tj *dj: This became alveolar affricates in West Slavic and alveopalatal affricates in South Slavic. The outcome of some Hungarian towns such as Vencsellő < *Vętjeslavŭ seems to suggest a South Slavic reflex. Therefore, the outcome of *tj *dj is /t͡ʃ/ /d͡ʒ/. As this occurs, the clusters *stj and *zdj behave differently, as they might always have been affricates. I’ve decided to treat them as single *tj *dj and have the outcome be the same.
Palatalization: While I am waiting to see what outcomes in Hungarian may suggest, I am leaning towards having a general palatalization before front vowels and then depalatalizing them like in Czech and Slovak. I’ve included a similar type of palatalization in Pelsodian.
I'm leaving it at this for now. I invite my two Croatian friends, Zekoslav and Click, into this thread to share their opinions, as they have a fair amount of knowledge and background on Slavic languages while I am an infant stumbling around. Nonetheless, I've learned a lot about the Slavic languages in the last couple of weeks, and hopefully it'll show off here.
ъ ь > e o (alternating in strong syllables only)
ъ ь > ∅ (elsewhere)
*k *g *x before a front vowel > tʃ ʒ s
*k *g *x after a front vowel > ts z s
*kj *gj *xj > tʃ ʒ s
*sj *zj > ʃ ʒ
*tj *dj > c͡ç ɟ͡ʝ
*stj *zdj > ʃc ʒɟ
*kv *gv > tsv zv
*eRC, *oRC > ReC, RaC
*CeRC, *CoRC > CRēC, CRāC
*tl *dl > l
Common Slavic to Old Pannonian:
∅ > v / _u
ts before v > s
c͡ç ɟ͡ʝ > t͡ʃ d͡ʒ
ʃc ʒɟ > ʃt͡ʃ ʒd͡ʒ
Old Pannonian to Middle Pannonian:
s z before v > ʃ ʒ
Middle Pannonian to Modern Pannonian:
Old Slovene: “Iazze zaglagolo zlodeiu. Iuzem iego delom. Iuzem iego lepocam.”
Modern Slovene: “Jaz se odpovem zlodeju, in vsem njegovim delom, in vsem njegovim lepotijam."
English: “I renounce the devil and all his works, and all his ornaments.”