Rehgar has been my favorite healer for a long time, but he's not so popular now. One of his biggest problems though is that he is quite ueeless as a healer at early levels. If someone is dying, Rehgar can't really change that before he gets Earth Shield. The heal is rarely enough. He is also locked into a single build early game for 90% of games.
Level 1 - The mana cost reduction on Chain Heal is absolutely amazing for him. I take Colossal Totem with an Illidan on the team. I think you guys know that Healing Totem (and Ward) have been overnerfed to the point that no one takes them. 1 Valla multishot scales perfectly to always one shot healing wards :)

Level 4: Feral Heart. You will have huge mana issues without it. Chain Reaction would be the only other good option, if Rehgar had better base mana regen. Shadow Wolf basically useless - easy to see and cancel. My suggestions here: buff Rehgar's base mana regen / give him access to Conjurer's Pursuit at level 1. Make Shadow Wolf like Tassadar's Dimensional Shift, 1s duration.

Level 7: Your usefulness as a healer is greatly hurt if you don't pick Earth Shield. People WILL blow up your assassins before your Ancestral Healing affects them if you don't pick it. I honestly think that Earth Shield should be included in Rehgar's base kit somehow. Either as an extra button, or simply baked into Lightning Shield. Cleanse is the only other option we have in this tier, simply because we need Earth Shield so much.

All the rest of the talent choices I think are very well balanced and fun to play with. You can go for Lunge, 90% slow and Rewind, which is absolutely amazing vs assassin heavy teams or the maximum healing build which is also great fun. I think that Rehgar only needs help with his early game.

I think that including Earth Shield in his base kit would completely fix his early game problems, as well as making the talent choice at level 7 more fun. His base kit is way too weak at low levels, even though it becomes better and better starting with level 13.