Primal Amulet Four-mana rocks need to offer a substantial reward given how slow they are, which Primal Amulet does
And it also divides opposing resources some - In my experience, dealing with a "go-wide" and a "go-tall" strategy is a lot harder, and Kellan allows you to play both in one concise package
Then Beast Within, all the artifact lands, all the untapped mana-fixing lands, etc
I've found all the sellers incredibly helpful, even some as far as putting in an extra card that they think would fit the theme (happened more than once you know who you are Rob)
Slivers If you thought another tribe should be here (other than humans), you are seriously deluding yourself
This sorcery is four, ramp any land (untapped and ready to be used) or destroy a land