It's definitely not a bad idea, my only concern is how you would go about implementing those changes. Something like that would require ASM hacking. Also, it would radically change the balance of the game, as you'd be able to have tank-like mages who are hard to kill, can kill groups of people, and can heal themselves up. I don't think the movement penalties would really effect them that much either, unless they can only move 1 tile at a time or something - at that point you have to consider fun factor.
As far as potion balancing, you could try making potions heal for a low % of max life, allowing them to scale throughout the game, but still be significantly weaker.
As far as potion balancing, you could try making potions heal for a low % of max life, allowing them to scale throughout the game, but still be significantly weaker.