I doubt they'll have more than 10 or 12 arrow skills. More than that and they get redundant in function or use. I'd be bored with a bunch of arrow attacks that differ chiefly by the type of elemental damage dealt. That's what runestones are for, ala Hydra.
Also, Bliz is somewhat limited in the effects if they want to avoid overlapping with Wizard functions. I think they want the chars to be distinct, so we're probably not going to see big Blizzard type AoE arrow effects, or things that mimic Slow Time, etc.
They also want to allow a lot of variety in character builds, so just having 25 different arrow attacks seems like a non-starter.
Here's the essential question: What distinguishes an arrow from a melee attack, or from a spell? Arrows do physical damage, like melee attacks, but they're less damaging and have greater accuracy requirements. They hit from a distance like spells, but they deliver primarily physical damage and the projectiles move faster.