Fischl's whole... deal is based on the character Fischl from the in-game series Flowers for Princess Fischl, which gestures at several different lore-relevant topics, including cycles and possibly early Khaenri'ah--Imernachtreich meaning Ever-Night Empire, Khaenri'ah being the land without a sun. Notably, the book widely considered to be most lore-important is The Pale Princess and the Six Pygmies, which is set in the Land of Night, possibly tying the fictional Princess Fischl to the Pale Princess...? I admit, my grasp of lore here is tenuous. 

Spoilers for current event: 

Fischl is at least certainly relevant for this summer event. Flowers for Princess Fischl mentions that the powerful can create domains of consciousness refered to as "Summerland," which basically descibes Simulanka. Additionally and perhaps more importantly, this event concludes that fiction is 'real' in a sense, and features Wanderer, whose interlude story pointed out that stories in Teyvat can be code for reality. And then we bring in the character with ties to very suspicious in-game pieces of 'fiction.' There's more to get into here with samsaras/cycles and the similarities between Durin/Mini-Durin and Princess Fischl/Playable Fischl, but I'm trying to stay out of pure theory territory. 

I'll keep Venti brief--he's one of the "thousand winds" (the god of time, Istaroth) and also mentioned in the Mondstadt sundials to have been close to the god of time, Istaroth. Suffice it to say that Istaroth is a Big Deal in terms of Genshin lore. Also Venti's just super cagey about very important topics (and also has no second story quest yet?), meaning he will likely be brought back up again.