My original idea was to increase mana regen when you had three Holy Power, though I'm not sure how well that would work. I've heard reports that our regen on the beta is pretty outrageous, but I don't know.
My new idea is a little more complex, but it fixes one of Holy's biggest problems (in my opinion): it's total reliance on big, straightforward heals, and its lack of absorbs (partially fixed through Mastery, I guess) and HoTs.
Maybe, once you reach three Holy Power, your mana-using direct heals (Divine Light, Flash of Light, Light of Dawn [Holy Shock has too much riding on it already]) also apply an HoT to your target. This effect consumes no Holy Power, and its strength is not tied to how much Holy Power you have; it only ever occurs when you have three.
This creates a decision: do you want to spread a few HoTs around over the course of the next several seconds, or do you want a large, free heal right now (and, now, free AoE healing)?
Ultimately, what I'm trying to think of are ways to give Holy Power (and the Holy paladin in general) more depth and decisions. What are some of your ideas for improving Holy Power or depth?