First i'd like to point out that while developers do take the player's choices into consideration, I highly doubt that flooding him requests is going to influence the design decisions that they've likely spent hours discussing and implementing. So getting all of the Bowser thread to bombard Sakurai with "GIVE US TAP FIRE PLEASE" will probably not be a wise choice of action; It's instead very likely to backfire.
That being said, the tap fireball idea is really cool, but ultimately I think just focusing on making the firebreath itself will be the more likely design decision that they make, and it seems like they've improved it already. However, if this projectile idea were implemented then It'd be a nice little addition but I doubt it'd do anything to force anyone to approach except for the characters that were already on the approach anyway. I'm pretty sure anyone else with a projectile would still be more than happy to chuck eggs, fire peanuts, throw bombs, shoot arrows, toss pikmin, e.c.t It might be a slight nusiance for them if he did have the fireball, but I highly doubt that'd be enough to make them approach us. Also, from a purely design-based standpoint, there are probably good reasons for firebreath instead of a projectile. From a player standpoint, if firebreath does get the proper buffs, then we probably won't even need the fireball anyway.