On topic, yes I think it could absolutely be done and Otto I is probably the right guy to do it. He was a Catholic hero and re-established the Roman Emperor title in a similar manner to Charles the Great. He had a universal claim to rule over all Roman territory and all Christians all Emperor, and so could demand the various Kings submit through a successful invasion.
The nobility would be integrated via the same feudal system in the East that also existed in France. It would not be difficult.
No, not Otto I.
But compare Robertings vs. Liudolfings, both starting from last pre-accession head.
Robertings:
- Hugh the Great. Born ca 898. Refused crown but inherited duchy in 923, age 25 - no minority. Fathered his heir Hugh Capet in 939, age 41. Died 956, age 58.
- Hugh Capet. Born 939. Inherited duchy in 956 age 17 - no minority. Fathered heir Robert in 972, age 33. Usurped kingdom in 987, age 48. Died 996, age 57
- Robert II. Born 972. Inherited kingdom in 996 age 24 - no minority. Fathered heir Henry in 1008, age 36. Died in 1031, age 59.
- Henry I. Born 1008. Inherited kingdom in 1031 age 23 - no minority. Fathered heir Philip in 1052, age 44. Died in 1060, age 52.
- Philip the Amorous. Born 1052. Inherited kingdom in 1060 age 8 - minority till 1066, age 14. Fathered heir Louis in 1081, age 29. Died in 1108, age 56
- Louis the Fat. Born 1081. Inherited kingdom in 1108 age 27 - no minority. Fathered heir Louis in 1120, age 39. Died in 1137, age 56.
And now look at Liudolfings:
- Otto the Illustrious - born no later than 840. Appointed as Duke in 880, no younger than 40 - no minority. Fathered his heir Henry in 876, aged at least 36. Died in 912 aged at least 72 - possibly 82.
- Henry the Fowler - born 876. Inherited duchy in 912, age 36 - no minority. Fathered his heir Otto the same year. Was elected king in 919, age 43. Died in 936, age 60.
- Otto I - born in 912. Inherited kingdom in 936, age 24 - no minority. Fathered his heir Otto in 955, age 43. Died in 973, age 60.
- Otto II - born 955. Inherited kingdom in 973, age 18 - no minority. Fathered his heir Otto in 980, age 25. Died in 983, age 28 - apparently malaria in Rome.
- Otto III - born 980. Inherited kingdom in 983, age 3 - minority till 994, age 14. Died in 1002, age 21 - apparently again malaria in Italy. No marriage, no children. Nearest kin were distant and discredited cousins of Ottonians, of whom Saint Henry won out after fight.
Yes, malaria in Italy and military involvement there mattered. Still - Otto I had the luck to survive.
Bulk of Robertings and the early Liudolfing kings cluster between 56 and 60.
So suppose that Otto I has the luck of his father, and lives till 1015. By which time Otto III is 35, needs no minority and has sons of his own.
Also meaning that in 987 Germany has an adult Emperor, not the weak Regency as OTL 983-994.
Otto I has interfered in Italy where House of Ivrea had usurped kingship. He took the occasion to unite Kingdom of Italy with Germany, and made it stick till 19th century.
Suppose Otto II is alive in 987, and takes the occasion to crack down on attempted usurpation of Hugh Capet.
Sure, there is the Carolingian claim. Charles of Lower Lorraine and Otto of Lower Lorraine. But OTL, Carolingians did die out with Otto of Lower Lorraine in 1012.
So, WI by 1015, Otto II is King of France as well as Italy and Germany, Capetings a footnote in history, and Otto III succeeds Otto II in all three kingdoms?