Experiment Kraj has all activated abilities of each other creature with a +1/+1 counter on it.
: Put a +1/+1 counter on target creature.
Drop creatures with potent activated abilities, seed them (and Kraj) with +1/+1 counters so Kraj copies their abilities, then go off by chaining tap/untap abilities. You ramp into Kraj on turn 4-5, build a counters/untap engine, and combo out once Kraj absorbs the right abilities.
Kraj's own ability puts a counter on a creature, immediately giving it (and itself) those activated abilities for free engines
Tons of infinite combo potential by absorbing mana-dork and untap abilities
Mono-counter synergy lets one Kraj turn an army into a toolbox of activated abilities
Simic gives the best ramp and card draw to support a 6-mana commander
Highly commander-dependent—losing Kraj cripples the strategy
Color pair lacks efficient removal and board wipes
Many key pieces are fragile creatures vulnerable to wraths and spot removal
Combos can be fiddly and telegraphed, inviting interaction
Legendary Creature — Ooze Mutant
: Put a +1/+1 counter on target creature.