There are a lot of Roman successor states.
This probably doesn't get all of them, but I made an effort (read: looked at a lot of Wikipedia articles). No distinction is made between actual successor states and conquerors. There are a few situations I refuse to tangle with- the many tiny states in Italy and Germany/the Holy Roman Empire are prime examples.
The Roman Imperial Line (27 BCE-1453 CE):
- Gallic and Palmyrene Empires (260-274 and 270-273 CE respectively)
- Western Roman Empire (286-330, 395-476 CE), see Odoacer's Kingdom
- Sub-Roman Britain and Armorica (407-7th to 9th Century CE?) see Frankish Kingdom?? followed by Kingdom of England (927-present???)
- Suebi Kingdom (409-585 CE), see Visigothic Kingdom
- Visigothic Kingdom (410-721? 475-721 CE) followed by Kingdom of Asturias/León (722-1230/1833 CE) (but see also Umayyad Caliphate) followed by Castile/Spain (1230-present CE)
- Burgundian Kingdom (411-534 CE), see Frankish Kingdom
- Frankish Kingdom (431-800 CE), see Carolingian Empire
- Vandal Kingdom (435-534 CE) followed by Kingdom of the Aurès (483-703 CE), see also Eastern Roman Empire and Umayyad Caliphate
- Soissons under Syagrius (464-486 CE), see Frankish Kingdom
- Ostrogothic Kingdom (469-553 CE), see Eastern Roman Empire
- Dalmatia under Nepos (475-480 CE), see Odoacer's Kingdom
- Odoacer's Kingdom of Italy (476-493 CE), see Ostrogothic Kingdom
- Mauro-Roman Kingdom (477-578 CE), followed by Kingdom of Altava (578-708 CE) (but see Eastern Roman Empire) and Kingdom of Ouarsenis (430-735 CE), see Umayyad Caliphate
- Eastern Roman Empire (286-330, 395-1204, 1261-1453/1475 CE), see Ottoman Empire
- Avar Khaganate (567-889? CE), see Frankish Kingdom/Carolingian Empire
- Samo's Empire (631-658 CE), followed by Kingdom of Moravia (833-907 CE)
- First Bulgarian Empire (681-1018 CE), see Eastern Roman Empire, followed by Second Bulgarian Empire (1185-1396), see Ottoman Empire
- Principality/Kingdom of Hungary (895-1301 CE), followed by Kingdom of Hungary (1301-1526 CE), followed by Kingdom of Hungary (1526-1867 CE), followed by Austria-Hungary (1867-1918), see also Ottoman Empire
- Duchy/Kingdom of Poland (960-1567 CE), followed by Polish-Lithuanian Commonwealth (1567-1795), partitioned among Habsburgs+Prussians+Russians, see post-1918 Poland + Lithuania
- Duchy of Bohemia (870-1198 CE), eventually part of Czechoslovakia
- Lombard Kingdom (568-774 CE), followed by Principality of Benevento (774-1053 CE), followed by Kingdom of Sicily (1130-1816)
- Rashidun Caliphate (632-661 CE), followed by Umayyad Caliphate (661-750 CE)
- Emirate of Nekor (710-1019 CE), see Caliphate of Córdoba
- Abbasid Caliphate (750-1258, 1261-1517 CE) (see also Mamluk Sultanate, 1250-1517 CE)
- Fatimid Caliphate (909-1171 CE)
- Emirate of Sicily (831-1091 CE), see Kingdom of Sicily
- Sultanate of Egypt (1171-1341 CE)
- Mongol Empire (1206-1368 CE) (i'm not dealing with all of this shit)
- Ottoman Empire (1299-1924 CE) (i should really do this justice because they're clearly the main Roman successor of the post-HRE period but)
- Emirate/Caliphate of Córdoba (756-1031 CE), fragments variously conquered by Aragon, Castile, Valencia, Almoravids, Almohads (this shit is so complicated it needs its own thing)
- Republic of Venice (697-1797 CE), followed by Napoleonic Cisalpine Republic/Republic of Italy/Kingdom of Italy (1797-1814)
- Kievan Rus' (879-1240 CE), followed by Principality of Kiev (1132-1471), see Lithuania
- Principality of Chernigov (988-1402 CE), followed by Grand Duchy of Lithuania (1236-1567 CE) and Principality of Ryazan (1097-1521 CE), see Polish-Lithuanian Commonwealth and Grand Duchy of Moscow
- Novgorod Republic (1136-1478 CE), followed by Pskov Republic (1348-1510 CE) and Great Perm (1323-1505), see Tsardom of Russia
- Grand Duchy of Vladimir (1157-1331 CE), followed by Moscow (1263-1547 CE), followed by Tsardom of Russia/Russian Empire (1547-1917 CE)
- Judiciates of Sardinia (1000-1324 CE), followed by Kingdom of Sardinia (1324-1861 CE), followed by Kingdom of Italy (1861-1946 CE)
- Sultanate of Rûm (1077-1308 CE), see also Ottoman Empire and Ilkhanate
- Empire of Nicaea (1204-1261 CE), refounded Eastern Roman Empire, see also Latin Empire and Republic of Venice
- Empire of Trebizond (1204-1461 CE), followed by Principality of Theodoro (1362?-1475 CE)
- Despotate of Epirus/Empire of Thessalonica (1204-1337 CE)
- Despotate of Morea (1349-1460 CE)
- Papal Line (30-present CE? 380-present CE)
- Carolingian Empire (800-888 CE)
- Papal States (756-1798, 1809-1870 CE), followed by Vatican City (1929-present CE)
- Middle Francia (843-855 CE)
- Lotharingia (855-959 CE), divided into 3 duchies of Kingdom of Germany/Holy Roman Empire (eventually parts of Swiss and Lower Countries)
- Carolingian Kingdom of Italy (855-962 CE), see Holy Roman Empire (constitutent of HRE 962-1801), see Italian Republic
- Lotharingian Upper Burgundy (888-933 CE), followed by Kingdom of Burgundy (933-1032), see Holy Roman Empire
- East Francia (843-962 CE), followed by Holy Roman Empire (962-1806)
- Napoleonic German states (1798-1813 CE)
- Anti-Napoleonic German states (1804-1871 CE), followed by German Empire (1871-1918) and Austro-Hungarian Empire
- West Francia/Kingdom of France (843-1792 CE), followed by First French Republic and First French Empire (1792-1814 CE), followed by Kingdom of France (1814-1848)
- Republic of Genoa (958-1797 CE), followed by Napoleonic Ligurian Republic (1797-1805) and annexed by First French Empire
- Duchy of Genoa (1815-1848), see Kingdom of Sardinia
- Levantine Crusader States (1098-1291 CE)
- Latin Empire and Greek Crusader States (1204-1261/1579 CE)
- Teutonic Order and Prussian Crusader States (1226-1561 CE)
- Roman Republics (1798-1799 and 1849-1850 CE)
Here's my grand unified Take. In 475, Julius Nepos (sent in 474 by Eastern Emperors Leo I and Zeno to depose the Burgundian-appointed Western Emperor Glycerius, who accepted Nepos' rule) was deposed by Orestes who put his son Romulus Augustus on the throne. Nepos fled to Dalmatia, and ruled as Emperor (recognized by the Eastern Empire) there until his assassination in 480. Syagrius, ruler of Gallo-Roman Soissons, did not accept Nepos and ruled Soissons as if it was a Roman province from 464 to 486, when his territory was conquered by Frankish king Clovis I. In 476, Romulus Augustus was overthrown by Odoacer- who was recognized as ruler of Italy by the Eastern Empire). Odoacer took over Dalmatia in 482, but the Ostrogoths- their king Theodoric also recognized as a Roman ruler by the Eastern Empire- conquered his domain in 493 and ruled it until the Eastern Empire took it in 553. In Africa, the Mauro-Roman Kingdom was effectively independent from 439 onwards. By 477, it had conquered swaths of the Vandal Kingdom and its kings claimed the title of Imperator (as did at least one king of the Kingdom of the Aurès. It allied with the Eastern Roman Empire to fight the Vandals in 535, and was largely conquered by the Eastern Roman Empire by 578. Successor states to the Mauro-Roman Kingdom and Vandal Kingdom persisted until 735, when their conquest by the Umayyad Caliphate was completed. This left the Eastern Roman Empire (which controlled both Ravenna and Rome until the 750s) as the only main Roman successor aside from the Papacy itself until the crowning of Charlemagne as Roman Emperor.
When Charlemagne was crowned Roman Emperor in 800, this began a long fourth Roman line in the from of his Carolingian Empire, which was divided in 843 into three parts. East Francia, under Otto I, generally claimed the most important portion of this line by establishing his Holy Roman Empire out of quite a few smaller states. The Holy Roman Empire, though generally not all that unified, maintained some semblance of existence for about a thousand years until Napoleon dissolved it in 1806.
The Eastern Roman Empire continued for its own thousand-year reign, interrupted in the 1200s by the Fourth Crusade but eventually refounded. Its remnant states were largely conquered by the ascendant Ottoman Empire, which claimed its title for some time.
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