|Statue of Domitian as Imperator, Vatican Museum|
The move towards autocracy started fully with Julius Caesar and never really ended. Those emperors who dismissed the senate as a body and were openly dismissal of its status (e.g. Nero, Tiberius, and Domitian) reaped the rewards of their disdain. They are portrayed as insane, tyrannical, incompetent, and worse by their contemporaries. As it happened, the writers were senators and, as we know, writers tend to get in the last word!
As Waters notes, by the time of Trajan, "the senate itself was now of little importance as an organ. Pliny complained bitterly about the triviality of its agenda under Domitian, but his own evidence for the subsequet period does little to assure us of the gravity of matters now entrusted to its wisdom" (387). Waters also observes that there was tremendous continuity in the main players of the governments of Domitian and Trajan. This is seen to further support Trajan's general interest in continuing policies started by Domitian; and a general disinterest in stirring the post. Waters (399) likewise observes that the evidence supports the view that Trajan treated Domitian's decisions as valid legal precedents. He also seems to have continued Domitian's general military policies, particularly with the Dacians. In the East we see perhaps a bit more innovation from Trajan: he is more aggressive where Domitian was inclined to peace (400-401).
This makes a lot of sense, especially since Domitian enjoyed the support of the military. The senate deeply disliked Domitian, and Suetonius relates their delight at the news of his assassination; but his assassination was not politically motivated, so far as we can tell. And he seems to have been reasonably popular with the Roman people. The senate could control the transmission of Domitian's memoria, via written historical and biographical accounts as well as their issuance of a decree of damnatio memoriae. But they couldn't re-shape the immediate reality in Rome. Indeed, upon the death of Domitian we see no widescale effort to undo his expansive building projects or to otherwise re-form them by Trajan (403). This is somewhat unusual if we think of Vespasian's actions after the assassination of Nero; and, much later, Constantine's appropriation of Maxentius's building projects in Rome.
That Trajan made few changes, apart from expending more effort to avoid directly hostile attacks on the honors of senators, makes perfect sense in this environment. The other tactic that Trajan used, as Augustus and others before him had done, was to select certain senators to receive special honors. So, while the senate as a body remained utterly devoid of influence on imperial policy, it was possible for individual senators to benefit as amici imperatoris, so to speak.
Waters highlights the extent to which, since the Sullan proscriptions and the devastating civil wars of the late republic (and again in 68-69 CE), the membership of the senate had been diluted and depleted. Trajan apparently continued to add members to the senate rather aggressively. According to Waters, the available data suggests that up to 40% of Trajan's new senators were provincials from newly-Romanized provinces of the Greek East (392). I agree with Waters that the absence of any comment on this policy is rather surprising. But it may also be that such actions by Trajan finally convinced some of the initially optimistic senators like Pliny and Tacitus that, in essence, they had another Domitian on their hands, but with the benefit that he spend large swaths of time outside of Rome on campaign. Their silence may well be the silence that accompanies tyranny rather than the silent approbation. Trajan was certainly far more savvy than Domitian in his dealings with the senate and in his willingness to remain more quiet about his assumption of offices. He understood that he did not need the constitutional powers of the offices and so was willing to leave them to the senate, for the most part.
Waters also notes that Trajan's coinage is similar to that of Domitian--and not at all like the coinage that appeared in the interregnum of Nerva (which featured such legends as libertas publica, roma renascens, fortuna, concordia). Under Trajan, the coin types tend towards abstractions: virtus, salus, fortuna, aeternitas. He also followed Domitian in being represented in the guise of Hercules or with the attributes of Hercules. And, finally, Trajan allowed himself to be assimilated to Rome's highest deity--Jupiter Optimus Maximus (note too the echo of Optimus Princeps). Trajan avoided issuing any coins commemorating Divus Nerva until after the death and deification of his biological father in 113 CE. After this point, he was keen to emphasize his divine lineage and also to deify other members of his family upon their death.
Waters makes a compelling case for viewing Trajan as a continuator of Domitian's political legacy. Certainly, Trajan was far more skilled than Domitian at handling the senate at Rome. He also spent less time in Rome, which may have eased tensions. But it's remarkable that he emerged as the Optimus Princeps and was memorialized as an emperor who worked productively with the senate for the good of the empire. In reality, he was every bit the autocrat Domitian was; and the evidence often cited for his revival of senatorial authority and powers seems to be wishful thinking from senators in the early days of Trajan's rule, at a time when experienced senators likely believed that they might be able to sway this novice emperor from a rather undistinguished provincial family.