So with Guilds spoilers just around the corner some of the guilds mechanics have been spoiled
Selesnya - Convoke (confirmed with BaB promo)
Dimir - Surveil. More or less the same as scy but you can put the card in your graveyard instead of the bottom of your library.
Boros - Mentor. When a creature with mentor attacks put a 1/1 counter on an attacking creature with less power
Gruul - Enrage? Yes the same mechanic from Ixalan
Golgari - Overgrowth? Rumored to be an ETB ability that scales with the number of creatures in your yard
Convoke is always a good ability, Enrage fits gruul perfectly but feels weird to reuse it so soon. Overgrowth seems interesting but might cause some balance issues. Surveil is a great mechanic gameplay wise but feels boring compared to transmute or cipher. Mentor is just boring. Im guessing the creatures will have additional abilities that trigger when a counter is placed on them but it really feels like Boros is getting shafted. (again)