The Fifth Doctor + Gogo, Master of Mimicry

The Fifth Doctor

Gogo, Master of Mimicry

How it works

1. At the beginning of your end step, The Fifth Doctor triggers.
2. Holding priority, activate any number of creatures you control by tapping them, adding at least 2.
3. Activate Gogo by paying 2 and tapping it, copying The Fifth Doctor's trigger.
4. Resolve the copy of The Fifth Doctor's trigger, untapping all creatures you control that didn't enter or attack this turn and putting a +1/+1 counter on them.
5. Repeat from step 2.

This loop relies on Gogo, Master of Mimicry acting as an engine to endlessly copy the end step trigger generated by The Fifth Doctor: Peaceful Coexistence. By using the mana produced from tapping your untapped creatures before resolving the copied trigger, you can repeatedly untap your entire board and grow every eligible creature infinitely large. Because The Fifth Doctor resets your creatures with fresh untaps and counter buffs each iteration, Gogo, Master of Mimicry keeps the chain alive as long as you have the necessary {X}{X} mana to invest into the copying process.