You could do both. CLW could go back to the old standard of 1d8+1/level capped at 1d8+5 which is sufficient for low level adventuring. Cure Serious and Cure Critical could do percentages, and Heal is full curing.
So a CLW spell cast by a 15th level cleric can heal 6 - 13 hp for a L1 fighter, and Cure Serious heals, say, half his maximum hp? Which one did you say was the more powerful spell again? This just creates a different conceptual issue than the current one.