Guys, I deliberately avoided spells that include "all creatures", "all permanents", etc. I'm sorry if I'm being mean right now, but I do not want that kind of ideas. I don't know why, but I just don't like them. "All" just sound too plain and boring, and I wanted something more unique.
Nature's Calling is NOT underpowered at all. With six of those, you get to have 18 creatures! Combine that with Frogs, Cockatrices, Adrenaline, Empathic Bond, etc... Massive swarm of creatures with massive healing. Nature's Calling is an extremely powerful rush card, not to mention its synergy with Reverse Time (get a 3/3 Crimson Dragon? rewind it and it becomes a REAL 12/3 Crimson Dragon!).
Adamantine is not OP. Procrastination and Dusk Mantle already reduce damage by 50%, and it cannot be broken by spells. Adamantine is only perhaps overpowered when combined with Jade Shield, which blocks all spell damage; it will reduce all damage by 2, then reduce all damage by 25%. But that is not OP, it's synergy.
And to me, it seems like you didn't find any 'useful contents' in something I posted...
Again, sorry if I'm being mean, but I didn't really find anything "useful" in your post. As I said above, I wanted to avoid "all your creatures XXXXXX" and things like that.