We won't 100% tie every champ to a specific seasonal theme, both new and VU/VGUs. Sometimes it will make a lot of sense so we will do it. For "Welcome to Noxus" LeBlanc was a key figure in the narrative and we felt like her old look was so outdated that the opportunity and the seasonal theme matched so it made sense to pursue. That won't always be the case though.
As Brice mentioned in the video we wont always have a seasonal theme be base Runeterra, it could also be a AU. In those cases the New Champions or updates can really be anything since region won't be a factor. We also might explore some seasonal themes that aren't directly tied to one specific faction.
Also champions can appear in the story of a different region, even if they are not from that region or faction