I have a couple of big problems with using this system instead of NG+.
First, the two systems are not incompatible. You can unlock extra content after the main story without preventing the use of NG+. So have both.
Second, the new content does not stay new. Quests unlocked after the main plot get completed once, then they go away just as quickly as the main story. Dungeons are usually repeatable, but why make just that one dungeon repeatable (and forcing the repetition of just the single small section) when you could make the entire game repeatable with NG+?
And, Third, I don't see much point. There's no reason the uber-dungeons couldn't be unlocked before you beat the game.
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I've always wondered why SE saves NG+ for the more 'experimental' titles.
It works best in games with branching storylines, so you can play through several times to experience everything. Chrono Cross did it best by combing those features with FF and Slow buttons.
I appreciated its inclusion in the later Suikoden games, combined with loading bonus content from the previous game in the series (something that was seriously bungled in FFVIII). Suikoden Tactics became a breeze using Lazlo (Hero IV).
As has been mentioned, FFX-2 has a great game mechanics clothed in a mediocre sequel.
I like the way the Tales series have a New game+. Throughout the game you earn grade based on how you fight, and at the end of the game you can start over and use the grade to carry over stuff from your old file or enhance the next game.