I think the card you're looking for is Harmonic sliver. Green (and white), cheap (costs 3, transmute!), and it also nails artifacts. As for Twilight shepherd, I'm no fan of it. To make good use of it you need to have a lot of creatures on the board. And it costs too much, and it's not green or blue. Persist is a nice ability, but you don't have anything to sacrifice persisters to. But if you want to, take a look at Kitchen finks.
Angel of Salvation is a nice beater, you could run some convoke if you have some way to generate a bunch of tokens. I doubt the deck needs it though. If you really want to change the deck, run the Reveillark combo. But the deck is highly synergistic as it is, and people don't like infinite combos. If you play it you'll find what cards you like and what cards you don't like. I can imagine the Ashcoat bears and Zealous guardians will come out at some point. These cards are synergistic within the deck, but crap otherwise. Well, not crap, but just not that good. Perhaps more Qasali ambushers or Whitemane lions?
Some cards you could look for:
-Mangara of Corondor. Untap, bounce, RFG, reiterate? Costs 3, transmute! Works nice with Karakas.
-Gilded drake. I just saw one for 4.50 euro in an expensive shop, should be cheaper elsewhere. This card is evil in this deck.
-Heartwood storyteller. This could be interesting with all the creatures you play. Especially nice in multiplayer. Costs 3, transmute!
-Mystic snake. Do I have to explain this?
-Eternal witness. Heh. Costs 3, did you notice that?
-Gaddock Teeg. Might be out of your budget, but together with the Meddling mage you can hose a lot of decks. Does not cost three though
But everything that does cost 3 isn't countered by him.
-Sunscape battlemage. Costs 3, transmute! And budget.
-Wall of roots. Mana acceleration is good. And you're going to have problems with really fast decks. That could be a weakness. If it is, I'd run Swords to plowshares or something like Daze. Plus more Eternal witness. Perhaps Qasali ambusher is good enough for the purpose of stopping weenie hordes.
-Wall of blossoms. Elvish visionary with a big butt.
-Voidmage husher. Can save you. I'd run 1.
-Mulldrifter. Carddrawing can't be bad.
-Plaxmanta. The right colors and a great card.
-Jotun grunt. Graveyard hate. Or:
-Loaming shaman. Costs 3, transmute!
I know I already mentioned some of these, but I'm getting enthousiastic about the deck so I just mentioned them again
Anyway, I'm considering to build this (kind of) deck myself. I have a survival deck laying around that I don't really like, I could use that as the basis of this deck. I also have 4 Force of Wills that are collecting dust for quite some time, these could go in as well. I'm not saying that you should run these cards, but they do fit in the deck and I happen to have them. Your deck is fine as it is. It can grow from here.