Nabataean Kingdom as a Vassal State to the Roman Empire

When the last Nabataean king Rabbel II Soter died his kingdom was annexed. How and why the Nabataean kingdom was annexed is unclear, but there is evidence of minor military skirmishes. Aside from that there wasn't much resistance to the annexation. The Romans initially did not want to govern Nabataea themselves, it already had close relations with Rome.
Some of the effects of the annexation:
  • Greek officially replaced Nabataean Aramaic as the language of the administration, and Latin (and Greek) as the principal languages in the army
  • A road was built straight through it linking Syria to the Red Sea (Via Nova Traiana)
  • Petra began to decline gradually, partially because Bosra was the capital but also the caravan trade had already been declining. However, it still flourished in the early years of the province
In this scenario Trajan does not annex the kingdom. It may have been initially part of his master plan to defeat the Parthians, however he decides to spare it as it is friendly towards Rome. Or the Dacians put up a fiercer fight than Rome expected, and attention is instead directed there. Rabbel II had an heir, Obodas IV. Obodas IV would be the new king, ruling from 106 to 136 AD. Obodas IV, well aware of the declining caravan revenue following the shift to maritime trade, invests in the port city of Aila and cooperates with the Romans in the building of a road linking Syria to the Red Sea. The Nabataeans support Trajan in his Parthian expedition of 114 by helping the Romans cross the desert and supplying them with camels for the march and as auxiliary cavalry. Trajan dies in 117, his great expedition against the Parthians a failure. Trajan’s successor, Hadrian, gives up the conquests in the east. He withdraws the Roman army to the Euphrates line while suppressing the Jewish revolt that had broken out under Trajan with the help of the Nabataeans, long-time rivals of the Hasmoneans. The Nabataeans once again aided the Romans during the Bar Kokhba revolt. The kingdom's consistent support of the Romans means its status as a vassal is not disturbed. Obodas's remaining years are spent fortifying the frontiers of the kingdom in the Hejaz and Jawf and securing alliances with neighboring tribes such as Thamud.

Obodas was succeeded by his son Rabbel III (136 - 160), who maintained good relations with the Romans. He spends his time fortifying Wadi Sirhan and fighting off intrusions from, or somehow pacifying, nomadic tribes such as the Ḥawlat and Ḍyf. Further north he worked out an agreed on boundary with Roman Syria a little to the north of Bosra. Rabbel III, being a traditionalist and something of a romantic, decided to revert the capital of the kingdom back to Petra. He saw Petra as having more dignity and antiquity than Bosra and also wanted a capital that was closer to the Red Sea trade. This slowed the decline of the city even as Palmyra grew in importance and attracted the Arabian trade away from it.

The efforts of Obodas IV and Rabbel II were not enough as later Nabataean kings hungrily looked at controlling Roman Syria. As the Crisis of the Third Century unfolded, the Nabataean king strategically annexes key territories along the trade routes that linked Mesopotamia to Egypt including southern Syria and Egypt's eastern delta. Leveraging their strategic location and alliances with local tribes, the Nabataeans renegotiate their relationship with Rome to secure concessions and allow for greater autonomy and control over the newly acquired territories.
While the Romans didn't necessarily want to govern the Kingdom, it really was better this way for them and it was long-standing practice for them by this point to intervene at swordpoint in successions. I guess that with a very long string of sixes you could get that, but the likely consequence is that if the Empire doesn't crumble, it comes knocking for the upstart that threatens Syria and is likely to entertain good relations with the Persian arch-enemy.