In every expansion so far, each champion added to a prior region is intended to synergize with the new region is some way. There are some examples that don't play out in practice, but the intent was definitely there (LeeSin Fiiz and Jarvan Renekton for example).

In Rising Tides, all cards were released together, so no one was missing their synergy pairing. Call of the Mountain masterfully had some champions that overlaped into two different exisiting regions, so although Riven didn't exist at its launch, Taric still had Lulu, and although Shyvana didn't exist at ts launch, A-Sol still had Trundle (and the pairings of Soraka Kench and Zoe Victor released together). It was not until Empires of the Ascended that we got the first example of a Champion releasing without an other region counterpart. Examples of this include Azir and Zilean.

Zilean was sort of a joke until Ekko released, and I'll say the archetype still isn't performing well, but Zilean didn't really find a footing until the release of Ekko. Azir, on the other hand, was doing just fine before the release Irelia, and once Irelia released, he started becoming an issue due to being too good.

I think is is very likely that we will see some intended Bandle City support for Sion, meaning we arn't even seeing Sion's full archetype yet. Though, Sion is not in a bad place like Zilean was, I've seen comments on the sub-reddit saying he should be nerfed. I can foresee the same situation that happened with Azir happening with Sion, where a champion who performs well even without their intended synergy, becomes overbearing once they have access to their intended synergy.