I'm hopeful that spell power will be eventually be used for more than just damage. There are alot of spells out there that don't directly deal damage (charms, buffs). Wouldn't it be nice if spell power affected these as well? In this case, the % may not make sense.
Ideally, it would be nice if each spell could have traits that could be affected by spell power (probably individual for each spell). Not only would that allow better tweaking for balance, but it would also allow for spells to be given unique niche function. There's always the argument that this complicates things, but honestly, anyone who has played a caster has to do a fair amount of research on each individual spell anyway to figure out what works well and what doesn't.
Here are some examples of spells and Spell Power effects. Things in brackets could be auto-calculated and provided as a number:
Bull's Strength: Provides +4 Strength enhancement bonus (addtionally provides [ 2 * Spell Power / 100 ] Strength enhancement). That would make this spell useful beyond the low levels that it is currently.
Hypnotize: Affects targets for 6 seconds plus [Spell Power/20] seconds
Hold Person: Held targets are subject to 50% additional damage plus [50 * Spell Power /100]% for a total of...
Acid Arrow: Target is affected by 3 ticks of acid arrow + [Spell Power/20] additional ticks
Hovering over the spell should provide you with the numbers based off your current build/equipment. There are tons of ways that spell power could effect a spell (increased save, greater spell penetration, more ticks for DOTs, greater range, larger AOE, additional targets).
All that wishing aside, what I would REALLY like to see is that when I hover over a spell it provides me with it's base damage and then in parenthesis next to that the modified damage based off of spell power.