How about a singleton copy of any Version of green Garruks? It's on flavour and they're all very powerful and synergystic with your deck. (
Garruk Wildspeaker
or
Garruk, Primal Hunter
.)
A mono-green beast deck is not going to win any tournaments anyway, so maybe just mix it up little. It doesn't really need to be all playsets. It will get boring fast, if every card is a 4x.
I second
Wirewood Savage
. Card advantage is important.
You may want to run more answer cards so you don't immediatley lose to random powerful stuff like
Form of the Dragon
,
Teferi's Moat
, planeswalker or troublesome lands.
Mold Shambler
helps you do that without forcing you to lower your beast count. Depending on your metagame, or if you play more multiplayer
Molder Slug
can be very worthwhile, letting you deal with artifacts repeatedly.
Some of the new Bloodrush creatures from Gatecrash could be worthwhile. E.g.
Slaughterhorn
, as it can protect one of your creatures from burn, or generally make your opponent think twice about blocking when he already has seen you play it in another game. And since it's a beast, too, in case you don't a pump spell, it's another dude that lets you draw a card or gain life with your beast enablers.
Versatility is key in my opinion.
By now, I probably don't have to explain to you why I think that you should really be running a couple of
Krosan Tusker
s. They are great at every stage of the game.
Also,
Thragtusk
, which you probably thought of but I'm mentioning it in case you forgot and have the pocketmoney...
To make space, take out some of the dudes, that actually don't do anything interesting besides being dudes and reduce the numbers on some of the really expensive ones (cmc 6+). So no Durkwood Baloths and -1 or -2 Garruk's Horde. Rampging Baloth, as has been mentioned, doesn't really do enough in your deck right now.
Maybe lower your entire curve significantly so you can skip (some of) the manaelves/birds?
Also, 24 Forests is boring.
1x
Mosswort Bridge
1x
Oran-Rief, the Vastwood
?x
Cavern of Souls
A small red splash? Birds of Paradise and 4xFire-Lit Thicket, which can provide untapped painless 
on turn two would allow you to play red things without hurting your monogreen base in the slightest. You could consider 1 or 2 Contested Cliffs or Kessig Wolf Run also ghor-clan rampager becomes available.Basically this would let you play more "spells" (in the form of lands), without hurting your creature count. It adds consistensy and variety.