True they could just add a bunch of random NPCs with no back story and have the WOL just tag along as an adventurer. But the issue with this is that most people know who the WOL is. We saw this in parts of the questline itself. You could add a single character and could possible give enough scope to get a sense of who they were, but i think, WOL is too well known to just "drop in" and join another team of adventurers. If said characters joined as part of a expedition, then yes i could see that happening, but ultimately, i still think 6.1 and 6.2 is about wrapping up loose ends while also laying the stones for the next story. Thavnair's problems were neatly dealt with. 6.2 will probably do something similiar with Garlemald. Sharlayan does not have anything to "wrap up" so to speak, but i wouldn't be surprised if it doesnt serve a purpose in 6.3. This would leave 6.4 and 6.5 to set up and include the initial characters for 7.0 with more being introduced later. Again i honestly dont think they will replace all of the scions and those we get, we likely already know, (such as jullas or Sicard).
Yes it isnt impossible for them to have added new characters in 6.1 - but moving on wasnt the entire point of the 6.1 story, the focus was wrapping up Thavnair, providing hooks for the new story going forward, Provide some light comic releif to balance the overly emotive and heavy Endwalker MSQ and provide a thread into 6.2 - it did all of these well and honestly, i dont think they had adequate scope to plausibly add another character to focus on in 6.1 - When the trajectory is set, then yes i could plausibly see one added them, but currently it does IMO feel like they are wrapping up the scions for Retirement prior to adding any new characters who will follow us on our next journey. Besies if they are going to add a character will it really be a random adventurer from the bar in Thavnair or will it be someone central to the story of 7.0? I honestly think the latter is more likely.