Echo Robin

Mown's is neat but imagine someone using it to cast the same removal twice per untap for the rest of the game.

Hello World.

Altimis.

They'll run out of creatures eventually :q

I quite like TPmanW's for curious reasons, so voting for that.

Mown card+time warp could be problematic, any chance we could change the wording a little?

That is fair, although I don't think you can make it elegant at that point.
Easiest fix is making it permanents only, which also makes it a whole lot more green, so that's not too bad.

Other more exotic options:
during your turn, for each card type, you may cast a card of that type from your graveyard not yet cast this way.
You may cast a card from your graveyard each turn. If that card would be put into a graveyard this/that/on the same turn, exile it instead.
You may cast a card from your graveyard each turn as long as it doesn't share a name with a card already cast this way.
You may draw cards from your graveyard (as thought they were the top card of your library). (Then you will at least need a win-con in play or some draw engine.)
If you would draw a card, you may put a card from your graveyard into your hand instead. (Just a more general wording of the previous one.)
At the beginning of each opponent's upkeep, choose a card in your graveyard. You may cast that card until that player's next turn.

I would just like to remind everyone that combos exist in the game of Magic the Gathering, and are not inherently evil.

I'm mostly worried about a ban in multiplayer commander since both this card and time warp effects are probably going to naturally find a way in most GU decks, having a more exciting card for modern is an upside.Maybe we should worry about the exact wording after the round ends if it wins?
I'm voting for mown for now.

Confused's Summoner's Circle

While this is obviously true, there is some degree of acceptability as to how easily they should be possible to assemble. Being a creature brings a fair share of drawbacks, but it curves pretty easily from turn 4 into infinite on turn 5.

I hadn't thought about the Time Warp thing. I change my vote to Hello World.

I vote for
Cato:
Greengrove Elder
Creature - Treefolk Shaman
At the beginning of your upkeep, put a +1/+1 counter on ~.
Remove 1 or more +1/+1 counters from ~: search your library for a creature card with converted mana cost equal to the number of counters removed this way and put it onto the battlefield.
2/4

thatmarkguy, what's his name, Mark. Yeah, I vote for him.

I vote Restoration Spirit

Current Tally:

Hello World: 2
Mown: 2
altimis: 1
TPzombieW: 1
Confused: 1
Cato: 1
thatmarkguy: 1

And we have a tie! Echo_Robin, please cast the deciding vote for any of our seven contestants.

Restoration Spirit, but with a slight wording change if possible.

OK, Mown takes it! Restoration Spirit has been added to the spoiler with the following wording:

During each of your turns, you may play one card from your graveyard. If that card is an instant or sorcery card, exile it as it resolves.

Wording references: Karador, Ghost Chieftain and Buyback.

Well, that's two changes, since it now only works on your turn.
Anyway, I think you're better off just restricting it to permanents. It 1) Makes me less butthurt that the elegance is ruined (although the Karador wording already does that, which I technically knew about) and 2) Is arguably more flavorful, given the name.

Anyway.
Green common. In some form or another provides two or more types (Artifact lands, enchantment creatures, rampant growth, etc).

Seep through the Cracks –
Instant
Put a 1/1 green Ooze creature token onto the battlefield.
Symbiotic (Spells you cast this turn that share a card type with this card cost less to cast.)