I think Luigi is fine the way he is. I would like to see him throw thunder balls instead of fireballs though. Just think that would be kinda cool.
Also, this may be unpopular, but I actually want him to be unlockable again. Why? Because, well, think about it. It just makes sense for him. I mean, one of Luigi's prominent traits in Mario is that, well, he's always cast aside and forgotten in favor of Mario. He's always living in his shadow, never getting his own piece of the cake, so thinking about it that way, being unlockable to me, is the best way to represent his status as the left behind shadow in Mario games.