I personally dislike Incremental Growth. Reap What Is Sown is far more powerful. Why, you ask? First let's look at the CMC (Converted Mana Cost). RWIS is only 3 mana, compared to Growth's 5. Secondly, RWIS is instant speed, meaning it can be used as a combat trick, or to help your 3/3 survive a bile blight or lightning strike.
**Note: This below section takes this to a more aggressive strategy, and has no testing to back it. I would personally recommend the slower strategy explained later**
Centaur Battlemaster is another expensive choice here, it's presence on the board is very weak initially compared to other 5 cmc creatures in the format. It takes an additional spell to get value out of it, but by then you're either trying to close out the game with other threats, or stay alive. This does not seem to be a very control-focused deck, so I'd recommend sticking to the faster side. I personally think most heroic creatures won't work out in this deck, apart from the obvious Phalanx Leader.
I'd heavily recommend Fleecemane Lion, as it is a good early attacker, and becomes impossible to remove once it becomes monstrous, along with getting the +1/+1 counter bonus.
I wrote up a quick list here:
http://tappedout.net/mtg-decks/08-02-15-fyM-gw-counters/
Please note that the list was written without any testing. There were also many cards that can be added, but were not added in the interest of not exceeding 60 cards. Modify as you wish.
**Midrange Strategy**
Alternatively, you can go with a far slower strategy, abusing the Abzan's Ability to outlast their opponent. To do this, add in black for an assortment of removal spells to help stall the game. Add in Anafenza the Foremost for her attacking trigger, and maybe Siege Rhino if you find his drain necessary. Abzan charm would also be a must have in this case. In this slower strategy, which I believe you would enjoy more, the additional mana ramp would be much needed, and the slower cards you mentioned can find a home. Fleecemane Lion will still find a nice home here, as it is far too powerful here to ignore. Although I personally don't think Incremental Growth can stay, Centaur Battlemaster could make the cut. I won't write a list for this version of the deck, as I believe you already have an idea of how to go about this.