I'm going to bet on it being Daisy from Annie's new lore. There's no way they would just introduce her like that, give her a ton of motifs in relation to Annie, not to mention make her TOTALLY CUTE and have a potentially great design with some overhaul, and then have her be actually just killed off in the backstory. She was the only one of Annie's three family members (not counting Annie's witch mom, Amoline, and even Daisy's unknown biological father, but they'd be different stories) who didn't have her death actually shown, just having her be washed down a waterfall, which could easily lead to her coming back, maybe with an awakening of or gaining magical powers, of the flowery sort of course.
Now I'm sure many would have concerns about a colorful flowery little girl mage: she might be too much like Lulu. However, I say there's plenty of room for differentiation, especially in abilities, but also thematically as well. Lulu is a Yordle and her age is a bit vague, given her lore and her status as a species with a much longer lifespan than humans. Lulu also has Yordles' innate magic, which is why she does more flower-themed magic rather than actually making flowers grow. Plus, there are plenty of champions with similar themes that manage to be different from one another. A prime case of this, I think, is Tryndamere and Olaf, who are so similar in their themes, appearance, in-game roles, and even abilities that I confused them when I first started playing League. However, taking a closer look at their lore (and abilities/specific in-game roles) defines them as much more distinct characters (although I think Olaf could use a lore update). If Riot can make champions with so much thematic similarity feel distinct and unique, I'm sure they could do the same with Daisy versus Lulu as colorful, whimsical, little girl flower mages (yes I know Lulu is a support, but her secondary role is a mage and she plays much like one).