Kaiser and Omega merely forced you to reuse it, but as I've said before there's little more boring than just giving everything charm status.
I dunno about the difficulty; granted the movies only showed two different battles, one for each dungeon, and it was nothing more than a character landing a killing blow on an enemy. I don't mind the challenge being derived mainly from puzzles and the occasional boss, but I would like some nasty enemy encounters (especially now that they're not random in CT; you kill a rotten little creature, and you're guaranteed not to fight it again until you meet a formation with it somewhere else on the map. None of that three steps, LOL HELLO AGAIN crap.)
The dimensional vortexes also look like they'll be much more fiendish than the Lost Sanctum, which by video is given the appearance of a glorified fetch quest with an obligatory boss to wrap it up.
Here, Slowflake- someone showed me a page out of an article that briefly discusses the new dungeons (not really saying anything about difficulty however) and I'll repeat the blurbs verbatim:
"Halfway through the game, mysterious portals appear in prehistoric times and in the middle ages, both leading to an area called the Lost Sanctum. Consisting of a village, several field areas, and two full-fledged dungeons, the Lost Sanctum adds hours of fresh gameplay through multiple sidequests that intertwine between the two eras.
Three additional Dimensional Vortex bonus dungeons open up once you've completed Chrono Trigger's main quest. We don't want to spoil everything, so we'll leave it at this: the dungeons feature new items, enemies, and bosses, and one of them even incorporates a fabled "lost" area that was reportedly planned for Chrono Trigger's initial release. Conquer all three Dimensional Vortices and you'll get the opportunity to square off against a brutal new final boss and witness a new ending (bringing the total ending count to more than 13.)"
So, yeah, nothing about difficulty, but they did mention the Singing Mountain and because the Lost Sanctum is said to open halfway through (as indicated by the video, apparently it's after you get the modified Epoch) I doubt there's much in the way of new enemies. In fact, in the only part of the vid showing a battle, it's against black two-headed serpents, which are probably the Ruminators you saw throughout Black Omen.