What was Emperor Palpatine's end game?

Unbelievably, I have just finally completed the entire Star Wars double trilogy of films, and while some episodes were less of an experience than others, I'm just delighted to have fixed a gaping hole in my pop culture collection. Yay me!

Still, I have a major question: what exactly was Emperor Palpatine's/Darth Sidious' end game, specifically in the original trilogy (Episodes IV, V & VI)?

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(Really, now? I think he's giving himself just a bit too much credit for "everything")

Okay, so in the prequel trilogy, I learnt of the Sith Rule of Two, where there's a Master and an Apprentice, with the latter usually eventually taking the place of the former through not-so-pleasant means. Now, I admire Palpatine's resilience just by the mere fact of him lasting this long and outliving two of his three apprentices (Darth Maul and Count Dooku), but surely he must have known it couldn't last forever, right? Was his strategy to keep discovering a younger and more powerful apprentice until the end of time?

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He shared his plan of converting Luke Skywalker to the Dark Side with his incumbent apprentice, Lord Vader, who surely must have known this didn't bode well for his position. Wasn't that a weird thing to do? And if, as seen in an earlier scene, he wasn't completely convinced Lord Vader had the strength of mind to kill his own son if it ever came to that, wasn't torturing Luke with Force lightning right in front of his already sentimental father a particularly bad idea, considering it was the final straw for Vader? If I really wanted to get all that hate to flow through Luke and convert him to the Dark side, I would instead torture and kill the weakened Vader and accomplish two things: get rid of an old apprentice - ensuring I outlive yet another - and also, irreversibly convert an enraged Luke.

I'd like to hear what you all think.

Disclaimer: If I mentioned anything from Star Wars canon that is wrong, remember I'm a noob. And yes, I know the events have to eventually fit the plot to give us all a great ending, but let's speak hypothetically.