Riot just published an article for their Legends of Runeterra Champion development process, and may have given away a little more than intended. In the first couple of sentences they mention two characters we haven't seen yet, and the particular phrasing appears to be confirmation that the Champions already exist.

Quote From Riot
From a celestial dragon who forged the very stars to a literal tree (actually, there are two trees), League of Legends has a wide range of champions. And when it came to bringing them to Legends of Runeterra, we had a lot to work with: four (or more) abilities, a backstory, in-game dialogue, art, player feedback… You get it. Sometimes we had a little too much.

The links in the article point to Aurelion Sol, an ancient, celestial dragon who forged the very stars in the cosmos, and Ivern, a half man, half tree who wanders the forest of Runeterra. The line that really stands out for me is "And when it CAME to bringing them to Legends of Runeterra...". This is past tense, so it indicates to me that they've already created them or they could have used existing characters in that sentence or phrased it differently.

Aurelion Sol isn't a huge surprise, as we already knew about new voice lines that were datamined last month. But his existence would confirm that Targon will be the next region introduced to Legends of Runeterra!

Ivern is more of a surprise, because we didn't know of any existing voice lines for him. However, it would make a lot more sense to see him, rather than Lulu, be introduced for the Ionia region as the next Champion.

All of our predictions for Rising Tides from the datamined article last month have been 100% correct. Do you agree with our predictions for the next region and Champions? We should get the next set sometime in July or August, so we'll know for sure by then. In the meantime we'll keep our eyes peeled for spoilers!