I hope that it has better gameplay than FE1. Either way, I would prefer an all-new story.
Why? You don't know the FE3 story (from what you read, I'm pretty damn sure you haven't played FE3), so FE12 should have an all-new story to you. You've never heard it before.
If I remember correctly, it has a plot-twist, far deeper characters and such. The story is definitely on par with FE8. It's lagging a bit behind pretty much all the other stories, though.
Well, it depends, I guess. To some people, Fire Emblem has become a bit too much of fighting super-natural enemies. FE3 is a bit more down-to-earth than the FE6-FE10 ones. Well, save FE6 and FE9, I guess.