MTG Salvation

Any extra turn cards combined with Jace, the Mind Sculptor [0] will result in infinite turns AND you'll get to draw a card as well for each turn.

As pointed above Scroll Rack for the same thing.

Along these lines..
Riverwise Augur can be used to try and get indefinite triggers by using "return to hand" or "play". Crystal Shard, Eratic Portal, Equilibrium, Cloudstone Curio, are some that come to mind.

Brainstorm, Dream Cache are spells that allow you to put cards back on top. If you can combine with Isochron Scepter and Brainstorm.
Graveyard retrieval like Archaeomancer, Mnemonic Wall, Salvager of Secrets again using the "return to hand" cards for indefinite use.
Mystical Tutor and Personal Tutor.

Library of Leng is an interesting card for this as well. All you need is a discard outlet.
But the rulings on Library of Leng are tricky, I'm getting clarification in the rulings section for help on this.
But I assume you can use things like Merfolk Looter to discard your spell to the top of library.

Also there are a ton of "draw a card and discard a card" instants and sorcery, and these are perfect for getting God-Eternal Kefnet ability to trigger in opponents turns. There are too many for listing them all right now. But these should be the bread and butter of getting an engine going period for the deck.

This commander reminds me a lot of Rashmi, Eternities Crafter where you are trying to set up spells for each opponents turns to get value. Rashmi can make it so you don't spend any mana, where as Kefnet is cost reduction.