When I first read the synopsis of The Crystal Empire (the two part premiere for season 3) I immediately got nostalgic happy feelings as it seemed to be borrowing a bit from an earlier generation episode called "The Return of Tambelon".
For your comparison here is the synopsis from The Crystal Empire:
Northern Equestria's Crystal Empire has reappeared after being afflicted with a curse that made it vanish for 1,000 years. Princess Celestia asks Twilight Sparkle and her friends to protect the empire from disappearing again.
Followed by a synopsis from Return of Tambelon:
The ancient city of Tambelon disappeared 500 years ago. Now, all of the unicorns mysteriously vanish before it returns and Grogar attempts to enslave the citizens on Dream Valley.
So needless to say I was both excited by the season premiere, and also a bit nervous about it. One of the creepiest things about Tambelon was that the unicorns disappeared there when they winked out (teleported), but this is essentially impossible in Equestria as unlike G1 unicorns only Twilight Sparkle has the ability to teleport in the current show. So I knew that if the opening truly were a reworking of Tambelon it would be quite different in some respects.
Cadence/Cadance shields the crystal empire from King Sombra
Luckily, I happened to enjoy the opener. It wasn't quite as strong as the season finale but I think in general that it is more difficult to write a season premiere than a season finale. The Crystal Empire has a lot of work to do in a very short amount of time considering we are introduced to a new territory of Equestria and a new breed of equestrian, the crystal ponies. I liked that Shining armor and Princess Cadence/Cadance were relocated to the crystal empire as it was nice to see them again and know that they weren't just around to have a wedding and disappear.
King Sombra, a unicorn with a black heart returns to attempt to reclaim the crystal empire.
In fact most of the complaints I've seen about this premiere is disappointment with the villain King Sombra. Now I think a corrupted unicorn is pretty interesting but he does spend a majority of his screen time as a gaseous cloud or laughing evilly. The traps he leaves for the crystal ponies, including a door that subjects anyone who touches it to their worst fear, are more effective than he is.
The crystal ponies have a collective amnesia when it comes to the last thousand years of enslavement.
The crystal ponies are the biggest difference between Tambelon and the Crystal Empire. In Tambelon the city appears under the power of Grogar and attacking Dream Valley so that the creatures (not just ponies) must band together to counterattack and defeat Grogar and the city. In the Crystal Empire Cadance and company already have a hold on the empire (even if it is shaky) so the salvation of the crystal ponies comes (mostly) from within instead of through an external force.
In Tambelon Grogar is defeated by the ringing of a bell to break his power of the city. In the Crystal Empire Twilight Sparkle and her friends must find the crystal heart that Sombra has hidden and activate it with the power of the crystal ponies to protect the city from King Sombra. In one version the city itself is a force of darkness and a threat to the valley and in another it is a precious jewel to be protected. In both generations the ponies are ultimately victorious.
Success changes all of the ponies (temporarily) into crystal ponies.