+1 Pillar.
Also Consecrated Ground!
Or some new cheap universally beneficial terrain. Or something that "undoes" terrain. Right now CG is the only terrain costing <8 mana that isn't Siren Only.
In addition to more Teleport, many more Lesser Teleport, and more Force Push, I could personally use a few more Force Wave and Repulse, partly because I play Domination and have several books available at a time. I have about 20 spare Minor Heals, though, so I don't have a need for more of them.
I'd tend to stay away from cards in the core set, aside from maybe Teleport, since (1) they're available in the Arena core set (e.g. if you find you always need more of them, you should probably buy another core set) and (2) they're in at least one of the Core Spell Tomes. Cards exclusive to expansions are exactly that... so Acid Ball, Meredia's Blessing, Pillar, Minor Teleport (!!!), Force Wave, Repulse, Leather Chausses, Disperse, Crumble, Arcane Ward, Mend, Brace Yourself, Glancing Blow, Redirect, Chitin Armor, etc, would be preferential candidates for such an "essentials" expansion.