Charming Prince has versatility! We can gain life, we can bounce a creature (Thalia's Lieutenant to get more counters, Reflector Mage to bounce another of our opponent's creatures, etc...), or we can scry to help set up future turns. Pretty good addition to the deck, I think.
Giant Killer in place of law-rune enfocer. This gives a removal and a creature with tap-down ability in a single card. Law-Rune Enforcer is one to tap down a creature, while Giant Killer is to do the same, so I think (???) Giant Killer is better because of the added removal, but only using one mana to tap down a creature (instead of two) is definitely better. I'll have to tinker with this one before I finalize it.
Knight of the White Orchid is a solid two-drop that has first strike and can help us get caught up or even ahead on land. Just remember to play this before you play your land if you’re only one land less than your opponent. Play Knight of the White Orchid, get the Plains, then play your land for turn. Even if we’re ahead on lands, it’s a solid 2/2 for two with first strike.
Kytheon, Hero of Akros
is a human that can trigger Thalia's Lieutenant, or get a counter from Thalia's Lieutenant before it flips. Yeah, unfortunately, that counter goes away when it flips, but then we get a pretty good planeswalker in Gideon, Battle-Forged! Moreover, the ability to give Kytheon, Hero of Akros
indestructible is pretty nice.
Reflector Mage is arguably one of the most annoying cards in Pioneer. Bouncing a creature and keeping our opponent from casting any copies their next turn—yes please! I’ve seriously thought about adding a couple of bounce-effect blue cards just to target my own Reflector Mage to get the ETB effect a second time. It’s that good.
Thalia, Heretic Cathar is a creature that started as a “3-of,” then a “2-of,” and now is a “1-of,” and I think that’s actually a good number. Pioneer is filled with nonbasic lands, so her ability to cause them to enter the battlefield tapped really annoys opponents, and having our opponent’s creatures enter tapped aggravates aggro players. That’s just a bonus. Additionally, she’s a 3/2 first stiker, and that’s not bad.
Thalia's Lieutenant is just a good card in a humans deck. It provides the normally weak humans with counters, and it can become quite large itself. That playing a second Thalia's Lieutenant while one is in play is even better as it doubles the +1/+1 counters put on the one in play! If opponents don’t answer this quickly, it can take over a game.
Thraben Inspector is arguably the best common 1-drop white creature ever printed. The 2 toughness is relevant, and the clue allows us to draw cards. An easy include in a humans deck.
