I agree with Siuloir to an extent. While I like the game, it's still an old NES game, which lots of room to improve. The best thing to do with FE2 is to adapt it to modern FE's system, while keeping it's story, characters, lore, and maps faithful to the original, with room to modify for a more tasteful experience.
You'd have to research quite a bit into the world map/battle maps. If you can't find them online, you would have to play the game to do so.
You'd also have to completely redo the story, stats, items, class variety, weapon system, and promotion system. And if you're putting in skills, there's a lot you have to do to balance that as well.
Not to mention you'd have to find someone to sprite Doma for you.
Though I like the idea of keeping the casting from health for the sake on uniqueness, I can't say I'd like to deal with it again. It's weird, because I like it and don't like it at the same time.
Mages become very powerful due to learning magic, but they become worthless after they drop in HP. Sisters, with their summoning, are extremely good units. Enemy mages are dangerous, but become an absolute joke when they run out of HP, especially bosses.
And you'd HAVE to change classes around, because not one playable character used axes, unless you're adding in OCs.