The canon novel Tarkin seems to explain Palpatine's long term plans best:
Save for Sidious, no sentient being in close to five thousand years had set foot in the shrine [below the Jedi Temple]. The room’s excavation and restoration had been carried out by machines under the supervision of 11-4D. Even Vader was unaware of the shrine’s existence. But it was here that they would one day work together the way Sidious and Plagueis had to coax from the dark side its final secrets. In the intervening years he had actually come to appreciate Plagueis for the planner and prophet he had been. Such perilous machinations required two Sith, one to serve as bait for the dark side, the other to be the vessel. Success would grant them the power to harness the full powers of the dark side, and allow them to rule for ten thousand years.
...
Eventually the dark side would grant him infallible foresight, but until such time future events would remain just out of clear sight, clouded by possibilities and the unremitting swirlings of the Force. He had made himself lord of all he surveyed, but he had much to learn. Actions meant to topple him from his lofty perch wouldn’t end with the successful containment of this most recent fiasco. But he would deal with any who chose to challenge him with the same precision he had applied to exterminating the Jedi. And he would not allow himself to be sidetracked from his goal of unlocking the secrets many of the Sith Masters before him had sought: the means to harness the powers of the dark side to reshape reality itself; in effect, to fashion a universe of his own creation. Not mere immortality of the sort Plagueis had lusted after, but influence of the ultimate sort.
As his Empire swelled, bringing more and more of the outer systems into its fold, so too would his power unfurl, until every being in the galaxy was held captive in his dark embrace.
Tarkin, p. 101, 242
Palpatine intended to learn such power from the dark side that he believed he would not only become immortal but create the universe in his own image -- a universe in which he ruled everyone.
Palpatine would not need to recruit new Sith once he and his apprentice discovered such secrets from the dark side -- they'd be immortal, and Palpatine would always be the Master. Palpatine attempted to turn Luke to the dark side only so that he could replace the damaged Vader with an uninjured and more powerful Skywalker. Had he succeeded in turning Luke, Palpatine could possibly attain his desired powers with his new and more powerful apprentice.
