Prototype Portal + Ugin's Nexus + Sarcatog

Prototype Portal

Ugin's Nexus

Sarcatog

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. Activate Sarcatog by sacrificing the Ugin's Nexus token.
3. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
4. Resolve the Sarcatog ability, giving it +1/+1 until end of turn.
5. Repeat each turn.