LSV's overview of decks to expect in the Pro Tour. This points toward why I think it's odd to discount red as a likely candidate for most-played color in the top 8, because it's a big part of not just aggressive archetypes but also Emerge. You have Depala and Veteran Motorist for vehicles, you have Fiery Temper and several others for madness, Unlicensed Disintegration is one of the best removal cards in the format, Kozilek's Return is a core piece of most Eldrazi decks along with red discard outlets, and Harnessed Lightning is a general all-star.
M-Files Kaladesh part 1
* Trusty Companion having "can't attack alone" instead of "can't attack or block alone" seems to be a consistent thing for white going forward, perhaps to differentiate it from red's version.
* They noticed the potential of Wispweaver Angel to go infinite with another copy of itself and a third card that cares about that, but they thought it would be fine at 5+ mana.
* Toolcraft Exemplar originally got a +1/+1 counter every time it was tapped if you controlled an artifact (still at W), but it would get out of control quickly.
* Fumigate originally exiled all creatures with converted mana cost 2 or less and gained you that much life for 2WW, but
Last Breath and similar effects are no longer considered to be in white's color pie. That is to say, it no longer punishes having small creatures, which I always thought was an odd fit for white anyway. Also, at that time, the back of double faced cards still had CMC of 0, so this was explicitly meant to hose those.
* With Tezzeret's Ambition, they talk as though they changed the mana cost from 3UU to 4U to make it not strictly worse than
Jace's Ingenuity, but they didn't actually do that.
* With Lost Legacy, Melissa DeTora was pretty insistent that people would think it was terrible, for the reasons people think it is.
* Cathartic Reunion was originally a 3R instant with the same effect, that was brought down to 2R, and then to 1R as a sorcery. When it was at 2R, Ken Nagle suggested that the card add red mana to your pool, and while that didn't happen, that shows that's one variant of this effect that they're considering.
* From Arborback Stomper's section, they deliberately made sure there were a lot of appealing uncommons that were powerful by themselves in any context. This is good for both new and returning players.