Germination is really the only reasonable L100 talent option. Even if you aren't double Rejuving targets often, it still buffs your total Rejuv throughput by 20% through the 3 second extension. With Rejuv accounting for 20%-35% of most Druids' typical total healing, it's a 4%-7% throughput increase.
Rampant Growth isn't really viable (except maybe on a fight with unlimited mana like Brackenspore), because it is way too mana intensive of a playstyle and because it absolutely requires Soul of the Forest to be usable (meaning giving up another HPM increasing talent in Incarnation). Moment of Clarity is way too cumbersome to use. For one thing, unless you are already casting a Regrowth when it procs, you won't get more than 2 Regrowths off during each proc. For another, you often don't want to (or can't) hard cast two Rejuvs in a row at the drop of a hat. If it procs when there are no appropriate targets to heal or when you have to move, or during Tranq, an MoC proc actually becomes a downgrade over a baseline OoC proc which you can sit on for up to 15 seconds to pick an appropriate place to cast Regrowth. That isn't likely worth taking a 20% nerf to Rejuv to take.