Prototype Portal + Ugin's Nexus + Atog

Prototype Portal

Ugin's Nexus

Atog

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. Activate Atog 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 Atog ability, giving it +2/+2 until end of turn.
5. Repeat each turn.