I think Riot could do a lot with SR to indicate something's coming.
- Change the weather
- Have something happen in the background
- Alter flora or fauna
- Perhaps Dragon or Baron could be "damaged" or altered from an encounter with a new champion or as a result of an event
- Maybe have a mysterious new item in one of the store keepers stashes
- If the store keepers ever do get VO they could mention something casually "Seems like the weather has taken a turn for the worst" (Tease Ao Shin) "That bow? A dark man came by looking for that" (Special Dialogue to Varus, hinting at future champ)
- The structures could be altered or perhaps a new monster could temporary take the place of one of the camps?
- Footprints, markings or equipment in the jungle (Similar to Bard)
I personally would like to see more written stuff like the additional lore released for Rek'sai. I think of all the stories and promotion items released in 2014 the Noxian cartographer's journal was the most interesting. Gave us so much information about the new lore, this next to the map of the black mists prior to Kalista were my favorite promotional items. NOTICE: both of these weren't videos or cinematics - two mediums which I believe has to work much harder to communicate similar messages. Plus videos take a lot more resources.
My least favorite promotional items of 2014 were probably the videos.
Trails of Poro, First Contact, the Mountain, the Shuriman Event - they don't add much more to their associated champions or the world their in.