I still think new or randomly generated maps would be better, especially if those new maps were where you found the keys. It's a good step toward creating more end-game content and replay value. Of course, if you want to forge all four rings, well, you may as well log out of the rest of your life and just plug yourself into. Then you'll have to beat three pairs of bosses to forge just one ring.

So that means, to defeat all three pairs you'll need to beat three of the Keywardens three times each and the fourth once.

The thing is, the portal only opens once and then has to be rebuilt by killing all theKeywardens again except for the last one who drops the plans for the Infernal Machine which opens the portal. You can use these demon parts to forge a Hellfire Ring, though there are four different types of Hellfire Rings, each with their own properties. There are three pairs in total, and each one drops a piece of a demon when they're defeated.

Anyways, once you open the portal you'll find pairs of bosses teamed up against you.

Infernal machine diablo 3