Metropolis part III would be about the laziest thing they could do.
This is my biggest issue. I don't want things to go back to the way they were, and making Metropolis III would be exactly that.
Metropolis part 2 isn't even a song. It's an album.
I mean, it's not like they did The Glass Prison about AA, and it became a smashing success, and so they made The Glass Prison part 2 (This Dying Soul) always about the AA. And eventually down the line they do The Glass Prison part 3 (The Root of All Evil), again about the AA.
Metropolis part 1 got the "part 1" tag just for the lulz, they're on record about that. The lyrics are random cool imaginery.
Metropolis part 2 was intended to be a 20 minutes epic; they constructed a concept album around that and they were, ehm, "heavily inspired" by the movie Dead Again for the storyline.
Metropolis part 3 has no musical (there's a song and an album, it's not like the AA saga) and lyrical (vague poetic stuff + concept album about reincarnation) sense. The Unforgiven III by Metallica made more sense than a possible Metropolis part 3.