So my thoughts here are that if we pull out
Boneyard Wurm x3, that means I've got 3 slots to fill.
I've still got 3x
Lotleth Troll in the deck
What about using something like
Crop Rotation or
Harrow?
I had
Dawntreader Elk in this deck before but feel like I can do better than that.
Edge of Autumn,
Font of Fertility is another option?
Growth Spasm creates a sacrificeable creature for a mana that I assume would go to the graveyard?
Harvest Wurm provides either an instant creature to graveyard or a land from graveyard to hand.
Terramorphic expanse seems to be what you were originally asking for?
I'm not married to any of the cards in this deck, so if you see a better combination as a result of shuffling out something else, let me know.
I'm also thinking that with the land return capabilities I might run less lands. So maybe I run 21 lands total instead?