It's quite simple. We'll do it LEGO style. Take each available talent tree (either WotLK or Cata, if you're in the Beta). Shake. Pick three.
To elaborate: Out of all the talent trees available, if you could freely choose three to create your own, 'custom' class, which ones would you choose? This is something that a few small budget roguelikes or diablo clones do, and it's fun to ponder if this would work in WoW as well.
Your starting class gives you the skills to work with. You may take for granted that all talents that enhance a class skill also automatically give you that skill. You must have at least one talent tree of your chosen starting class.
This one is a challenge for me! Hmm. I selected priest as my base class, because anyone who takes up this class in the lore would have a primary faith in the light, with an affinity for nature based solely on growing up in a rural area of Azeroth. My three talent trees would be holy-priest (for AOE heals), restoration-druid (for some powerful dots) and restoration-shaman (for some powerful single target instant heals). I would create a "super healer" class that has no options for being a hybrid. This class would have defensive spells such as Entangling Roots, Shackle Undead, Entangling Totems, etc, but very, very weak offensive capabilities. They would primarily be healers and cleansers.
Holy Priest TreeCircle of Healing (AoE), Spirit of Redemption, Lightwell. These spells provide AoE heals, a life saving guardian angel, and a freestanding healing well. The priest class also provides shields, a bouncing heal (Prayer of Mending) and one powerful HoT (Renew).
Restoration Druid TreeWild Growth (more AoE, but from a different school of magic), Tree of Life (boosted healing), and Swiftmend. With additional AoE and a boosted healing form, this healer is now REALLY getting into the role. Swiftmend provides a quick heal as long as a HoT is on the target.
Restoration Shaman TreeRiptide, Nature's Swiftness, Earth Shield, and Mana Tide Totem. The healing shaman tree comes loaded with goodies that would make this combination healer easily the most powerful class in Azeroth. Riptide is a fast single target heal that boosts haste for two spell cooldowns after; Nature's Swiftness makes your next spell instant cast; Earth Shield provides an instant heal every few seconds when your target is hit; Mana Tide Totem returns mana to you and your party with the touch of a button.
Lore-wise, this healer would follow the light but be respectful of nature. They would likely be found in a small, rural setting with smaller churches/cathedrals, similar to many people who live on farms in the countryside today. The nature-based talent trees would be necessary to survive enemies not found near the cities that have nature-based attacks, traits, and abilities. The light would be their spiritual following and the affinity for nature would be rooted in growing up in their current environment (countryside, rurual).
And since this is Pocket Heals, did you really expect anything different!?