Looking at the history of Valthungian, the period between Griutungi (the dialect of Gothic that was likely the main parent of Valthungian) and Middle Valthungian occurred mainly in northern Italy and some other areas of southern Europe, and borrowing from Vulgar Latin and its subsequent descendants was fairly rampant. One such borrowing that occurred around the time of Old Valthungian was the borrowing of the Latin genitive plural ending –ōrum to replace Valthungian’s earlier –ē and –ō, which had by that time were starting to undergo some final unstressed vowel degradation and were in danger of vanishing from the language completely, leaving a bare root.
Classical Latin –ōrum was borrowed from Vulgar Latin –ōrũ into Old Valthungian –ōru, which eventually became Middle Valthungian –oru and Modern Valthungian –aro.
Gothic NS: bagms ‘tree’, andeis ‘end’, handus ‘hand’, wata ‘water’ → GP: bagmē, andjē, handiwē, watanē
Griutungi: bagms, andīs, handus, watar → bagmē, andjē, handiwē, watanē
(early) Old Valthungian: bagms, eandijs, handus, vatar → bagmei, eandjei, heandjugei, vatanei
(later) Old Valthungian: bagms, eandijs, handus, vatar → bagmouruv, eandjouruv, heandjugouruv, watanouruv
Middle Valthungian: bagms, endîs, handus, watʀ → bagmoru, endzjoru, hendzjugoru, watɴoru
Modern Valthungian: bagmas, endis, handus, watra → bagmaro, enǧiro, henǧigaro, watanaro
Grammar sub-snippet: Why does watanaro still have so many vowels? Shouldn’t it be watnaro?
Yes and no. Watnaro is common enough in usage, but the retention of the unstressed /a/ here has to do with the development of Valthungian prosody, particularly throughout the Middle Valthungian period when a lot of vowels were jumping ship en masse, when the language came to have a much stricter iambic meter, so many unstressed vowels were retained and some were even added between two stressed syllables.