White Weenie seems like an obvious place to brew in Pioneer. There's a lot of 1-drop white creatures that are all somewhat interchangeable, but it turns out that many of them have the creature type Human, so we can support Thalia's Lieutenant.
White Weenie Humans Decklist (Pioneer)
4 Boros Elite 4 Dauntless Bodyguard 2 Knight of the White Orchid 3 Kytheon, Hero of Akros/Gideon, Battle-Forged 4 Mardu Woe-Reaper 1 Selfless Spirit 4 Soldier of the Pantheon 2 Benalish Marshal 2 Venerated Loxodon 4 Thalia's Lieutenant 4 Thraben Inspector 4 Declaration in Stone 4 Brave the Elements 16 Plains (331) 2 Castle Ardenvale 2 Selfless Spirit 4 Authority of the Consuls 3 Deafening Silence 2 Gideon, Ally of Zendikar 2 Hallowed Moonlight 2 Rest in Peace
Thalia's Lieutenant is the only real tribal synergy in the deck, but that's OK since the other Humans are all cards we'd want to play anyway or are roughly interchangeable with some of the other 1-drops available.
Mardu Woe-Reaper is a nice effect to have access to in the maindeck, since it could be devastating to certain opponents. We definitely don't want to sit across the table from this when we're playing either our Rally the Ancestors deck or our Arclight Phoenix deck.
The remainder of our 1-drops are mostly just there to beat down, but they all provide a bit of utility. Boros Elite is an above-rate creature the majority of the time, though it can be sideboarded out in games where we are expected to face a lot of sweepers and removal. The most notable interaction with Soldier of the Pantheon is its ability to survive Deafening Clarion. Dauntless Bodyguard is best when its protecting a Benalish Marshal or Thalia's Lieutenant from removal, but it can also give us a bit more insurance that we can keep a board against a sweeper. Thraben Inspector is our weakest attacker but ensures that we have more gas in the tank whenever we stall out on things to cast around turn four or five. Finishing out our long list of 1-drops is Kytheon, Hero of Akros. Kytheon is perhaps the strongest of the bunch since it can easily turn into a planeswalker, but since it is legendary we're only running three copies.
The only 2-drops to join Thalia's Lieutenant are a few copies of Knight of the White Orchid and Selfless Spirit. Knight of the White Orchid is an above rate creature that can lead to some great tempo advantages, but it's not completely reliable and our deck doesn't always have a use for extra mana. Selfless Spirit is often included in the sideboard of a deck like this as protection against sweepers, but in this case 1 copy is present in the maindeck which frees up a sideboard slot.
Brave the Elements is perhaps the biggest reason to play mono white as your aggro deck of choice. In the earlier turns it serves as a sort of Blossoming Defense type card against removal, and even protects against damage-based sweepers such as Anger of the Gods or Deafening Clarion. In creature matchups, it makes the game easy to win as you simply reach a stalled out game state and then make your creatures unblockable.
Declaration in Stone is one of the more powerful removal options available in Pioneer, let alone in Mono White. Gone are Lightning Bolt, Path to Exile, and the full potential of Fatal Push, so we're left looking for other options. Declaration in Stone gives us a hard removal spell that is at its best in an aggressive deck. The downside of giving our opponent a clue token is negated if we are aiming to end the game before they even have a chance to cast all of their cards, let alone invest two mana into drawing a card.
The curve-topping options include Benalish Marshal and Venerated Loxodon, some of the heavy hitters from the White Weenie deck of the most recently rotated Standard format. This particular list opts for a split of the two, as we don't want to find ourselves stuck with multiple copies of one in our hand. Venerated Loxodon does require us to take a turn off from attacking, but it quickly recoups that damage on the following turn. Both of these creatures are important for simply adding enough power to the table that a single Brave the Elements-fueled attack is enough to close out the game.
We're used to white having access to some of the best sideboard cards in Modern, and trend continues in Pioneer. Authority of the Consuls is particularly good against the Felidar Guardian combo, though it also has applications against opposing red creature decks. Rest in Peace is one of the most generically strong hate cards, shutting down several combo decks and any opponent who is relying on the big delve spells.