So I am ready to speculate. I think Naith Guide and the Tree People represent staple Silvan allies and events we shall see throughout the cycle, just like we had the Records attachments (Book, Map, Scroll and Tome) and the mono-sphere events in the previous one. Thus I believe we will see an Ally of cost 2 in each sphere and an event of cost 0 as well. Further, they will most likely have them two per pack (one for each but there it is getting harder to guess).
Also, since the third pack is supposed to feature no less than six Silvan cards, and Rúmil seem to be fitting there with his number, we might, in addition to the "regular" ally and event, see Haldir as a hero, and his brothers Rúmil and Orophin as allies in different spheres. Additionally, there would still have to be one more Silvan card in the box.
Finally, this is not much but since the trait boosting attachments were announced (and one spoilt already), I think it is not hard to guess that beside Tactics for Warrior, there will be Leadership for Noble, Lore for Ranger and Spirit for Scout.