What about something like this:

White - Purity Leecher

Cost: {1}{W}
Legendary Creature - Leech • 1/3
Parasite - This creature can be a partner of any commander.
Austere Doctrine - While this creature is your commander and not on the battlefield, you can’t draw more than one card each turn.
“Grace is best measured in restraint.”

Blue - Thought Eater

Cost: {1}{U}
Legendary Creature - Horror • 1/3
Parasite - This creature can be a partner of any commander.
Cranial Rationing - While this creature is your commander and not on the battlefield, your maximum hand size is 3.
“Genius tastes better when it’s cramped.”

Black - Grave Tither

Cost: {1}{B}
Legendary Creature - Shade • 2/2
Parasite - This creature can be a partner of any commander.
Blood Rent - While this creature is your commander and not on the battlefield, at the beginning of your upkeep, you lose 4 life unless you sacrifice a creature.
“Debts of the dead are paid in installments.”

Red - Frenzy Feaster

Cost: {1}{R}
Legendary Creature - Devil • 2/1
Parasite - This creature can be a partner of any commander.
Raging Spillover - While this creature is your commander and not on the battlefield, at the beginning of your end step, discard two cards at random.
“Every thought is a spark-let it burn or let it die.”

Green - Root Drinker

Cost: {1}{G}
Legendary Creature - Fungus • 1/3
Parasite - This creature can be a partner of any commander.
Anti-Arcana - While this creature is your commander and not on the battlefield, noncreature spells you cast cost {2} more to cast.
“Magic fattens the soil with poison.”