I could see a new champion, but I feel it's unlikely it's anything to do with "They are coming". The teaser thing or whatever it is is just to distant from the Shadow Isles colour scheme, sound style or general aesthetic. Maybe we'll see the Ruined King himself finally? One can wonder about such things...
I doubt they're doing an Evelynn update because we know Poppy and Taric are the one's in line for the big major updates and Evelynn herself is a HUGE undertaking that those two alone kinda force her to take a back seat. I could see them actually solidifying her lore background though, but I feel it'd be more about the legends surrounding her than her actual concrete origins. Think more "the first recorded appearance of Evelynn" rather than "She was the daughter of...".
Would love to see a new gamemode for Harrowing this year, I feel like it'd be grand! Hopefully not Hexakill again; I feel like they can do more with the Twisted Treeline than that.
I'd very much like to see some updates for the Shadow Isles champs in a low scope way but those who actually need updates need BIG updates. Yorick needs SO MUCH work done, Morde's basically a full VU at this point minus the VFX, Evelynn needs a MAJOR overhaul in every department, Hecarim could maybe use a bit of love? Maybe? Hell I'd take an expanded VO for him even though I know that's a lot of work. An actual full VU for Morde might not be out of the cards though...
The main thing I'm wondering though is how fucked Bilgewater is, and what the implications are. Just who's gonna be involved here to save Bilgewater? Or maybe not? Maybe they all die? Will Nami, Nautilus and Fizz make their mark on history? Will Gangplank and Miss Fortune die? For real this time for Gangplank? Will the Bearded Lady herself get involved? (Oh please Riot I'm really digging the Mother Serpent, would love there to be more on her) Either way I'm excited to see how fucked up Bilgewater is after the Harrowing's resolved.
**EDIT: **Ah I forgot, but I feel like I'd better mention it anyway - given Harrowings are an october deal (from out standpoint in terms of skins and stuff) September might be a very quiet month for lore. We'll have to see.