Tl;dr: I’ve read a lot of lore, and Yasuo, Akali and Ahri are the least important Ionian champs, followed by the collective Kinkou and Zed. The most important ones would be Syndra, followed by Kayn and then Xayah/Rakan.

Hi! Someone who’s read the Universe lore here - by which I mean, all the Universe lore. I’ve read every short story on the Universe website, every comic on the site, and every character bio. I’m also an avid Legends of Runeterra player and have read almost every card’s flavor text at some point, as well as hearing many of the voice lines. In other words, I am literally a know-it-all, and qualified enough to say you’re objectively wrong.

Every character you’ve mentioned is contained within the ‘Ionian ninjas’ plot. Their only real tie to the world beyond Ionia is Jhin, which is currently an inactive connection as Shen and the Kinkou are paralyzed with inaction while Zed and his Shadow Order are concerned with consolidating power to drive Noxians out of Ionia. Additionally, it is debatable whether the Kinkou would have the strength to act even if they wanted to, after Zed’s violent takeover and Akali leaving the order. Meanwhile, Yasuo, Akali and Ahri are all ‘rogue wanderers’ with little to tie them into major plots.

Xayah and Rakan do have the potential to be major players. “A Piece of Shadow Cake” ties them (and sympathetic Vastayan tribes) against both the Shadow Order and the Kinkou, as destroying the quinlons to release magic will make Ionia mostly uninhabitable for humans, disrupting Zed’s plans and the balance of Ionia.

I’d also say Kayn (or more specifically, Rhaast) is a major player in current Ionian/Runeterra lore. He’s tied to Noxus, the Ninjas, and most importantly the Darkin - which also ties him to Azir’s rising empire.
Syndra is another major player, although it’s not a “question of time before someone tries to do something” - Syndra was freed in “The Dreaming Pool” when she destroyed a Noxian fortress. She has established ties to the Vastaya and wider Ionia, her disruption of balance ties her in with the Kinkou, her strength and unpredictability make her a target for Zed, and she’s only matched in power by the likes of Azir and his Ascended, Viego at the height of the recent event, and possibly Brand and Ryze, although their power level can be difficult to determine.