Did Darth Bane have a plan for after his plan was finished? Would there be a rule of one? Or still a Rule of Two? Or even more than two Sith?
The Bane trilogy doesn't say much about Bane's plans after the Jedi were exterminated and the galaxy conquered. This goal was so far off (it took 1000 years before Palpatine destroyed the Republic) that Bane probably didn't have much of a plan himself. The closest example I can find says this (Bane's thoughts, my emphasis added):
Under his leadership the Sith had been reborn. Now they numbered only two—one Master and one apprentice; one to embody the power of the dark side, the other to crave it. Thus would the Sith line always flow from the strongest, the one most worthy. Bane’s Rule of Two ensured that the power of both Master and apprentice would grow from generation to generation until the Sith were finally able to exterminate the Jedi and usher in a new galactic age.
Star Wars: Dynasty of Evil, p. 9
This indicates that Bane always intended to follow the Rule of Two. He didn't indicate much about what would happen after the Republic was destroyed, other than that it would be a "new galactic age". The Sith likely would have ruled the galaxy as Master and apprentice, with the Master holding the position of emperor as well. The apprentice would become the Master and emperor once he killed his own Master.
Another passage explains Bane's reasoning for creating the Rule of Two (again, Bane's thoughts):
By its very nature, the dark side invites rivalry and strife. This is the greatest strength of the Sith: it culls the weak from our order.
The constant battling of the Sith since the beginning of recorded history served a necessary purpose: it kept the power of the dark side concentrated in a few powerful individuals. The Brotherhood had changed all that. There were now a hundred or more Dark Lords following Kaan, but most were weak and inferior. The Sith numbers were greater than they had ever been, yet they were still losing the war against the Jedi.
The power of the dark side cannot be dispersed among the masses. It must be concentrated in the few who are worthy of the honor.
The strength of numbers was a trap … one that had snared all the great Sith Lords who had come before. Naga Sadow, Exar Kun, Darth Revan: each had been powerful. Each had drawn disciples in, teaching them the ways of the dark side. Each had assembled an army of followers and unleashed them against the Jedi. Yet in each and every case the servants of light had prevailed.
The Jedi would always remain united in their cause. The Sith would always be brought low by infighting and betrayals. The very traits that drove them to individual greatness and glory—the unrelenting ambition, the insatiable hunger for power—would ultimately doom them as a whole. This was the inescapable paradox of the Sith.
Kaan had tried to solve the problem by making everyone equal in the Brotherhood. But his solution was flawed. It showed no understanding of the real problem. No understanding of the true nature of the dark side. The Sith must be ruled by a single leader: the very embodiment of the strength and power of the dark side.
If all are equal, then none is strong. Yet whoever rose from the swollen and bloated ranks of the Sith to claim the mantle of Dark Lord would never be able to hold it. In time the apprentices will unite their strength and overthrow the Master. It is inevitable. Together the weak would overwhelm the strong in a gross perversion of the natural order.
But there was another solution. A way to break the endless cycle dragging the Sith down. Bane understood that now. At first he had thought the answer might be to replace the order of the Sith with a single, all-powerful Dark Lord. No other Masters. No apprentices. Just one vessel to contain all the knowledge and power of the dark side. But he had quickly dismissed the idea.
Eventually even a Dark Lord would wither and die; all the knowledge of the Sith would be lost. If the leader grows weak, another must rise to seize the mantle. One alone would never work. But if the Sith numbered exactly two …
Minions and servants could be drawn into the service of the dark side by the temptation of power. They could be given small tastes of what it offered, as an owner might share morsels from the table with his faithful curs. In the end, however, there could be only one true Sith Master. And to serve this Master, there could be only one true apprentice.
Two there should be; no more, no less. One to embody the power, the other to crave it.
Star Wars: Path of Destruction, p. 237
Bane considered it a fundamental issue of the dark side that it had to be concentrated in a few individuals. Its "very nature" invites rivalry, which culled the weak but weakened the Sith as a whole by causing them to fight amongst themselves instead of their Jedi enemies. Bane also thought that the existence of too many Sith was a recurring problem that had caused the downfall of all the previous Sith Lords (e.g. Revan). On the other hand, a single Sith Lord would eventually die so "one alone would never work". Thus two Sith was the ideal number, and this would always be true because of the "true nature" of the dark side. Conquering the galaxy and exterminating the Jedi wouldn't change the nature of the dark side, so the Rule of Two would always be in force.