On first blush I feel like this "at least three cards with the mechanic" rule is pretty dumb, but it does explain a lot of the decisions in this set -- I think almost every significant absence is covered by either the three-cards-per-mechanic rule, overlap with Kaladesh Inventions, or a squeeze for space with a higher priority reprint. Since this set came out pretty good I guess it's forgivable, but I think they're still gonna have to up their reprint outlets a bit if this is how it's gonna work.
Yeah, I'm just guessing it's more Electromancer/Wee Dragonauts/etc. type shit than tribal proper.
Over/Under on Guttersnipe being in the deck is 2.5.