Dinosaur Egg

Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)
When this creature dies, you may discover X, where X is its toughness.

Retribution of the Ancients

{B}, Remove X +1/+1 counters from among creatures you control: Target creature gets -X/-X until end of turn.

Shalai and Hallar

Flying, vigilance
Whenever one or more +1/+1 counters are put on a creature you control, Shalai and Hallar deals that much damage to target opponent.

Unspeakable Symbol

Pay 3 life: Put a +1/+1 counter on target creature.

Master Biomancer

Each other creature you control enters with a number of additional +1/+1 counters on it equal to this creature's power and as a Mutant in addition to its other types.

Dragon Egg

Defender
When this creature dies, create a 2/2 red Dragon creature token with flying and "{R}: This token gets +1/+0 until end of turn."

Reyhan, Last of the Abzan

Reyhan enters with three +1/+1 counters on it.
Whenever a creature you control dies or is put into the command zone, if it had one or more +1/+1 counters on it, you may put that many +1/+1 counters on target creature.
Partner (You can have two commanders if both have partner.)

Uncivil Unrest

Nontoken creatures you control have riot. (They enter with your choice of a +1/+1 counter or haste.)
If a creature you control with a +1/+1 counter on it would deal damage to a permanent or player, it deals double that damage instead.