Exactly. The IVth is defeated in the Field Notes-epilogue of Bozja, which means possible future Dalmasca content would need to be tied to Bozja, and the type of content Yoshi-P mentioned fits with that more than an expansion since it’s easier to ask players to complete Bozja to access a side story than the main story.
The IVth legion exists independently to the status of the Garlean Empire as a whole since they deserted from it before the Save the Queen starts. The IVth then went on to control Dalmasca, Bozja, and Nagxia with Valnain as its capital city. So you can’t have any future content in Dalmasca, Bozja, or Nagxia, whether it’s an expansion or a side story, without first completing Save the Queen since that entire region is tied directly to the fate of the IVth, which is dealt with in that storyline.
While probably not as bad as doing Shadowbringers without doing Crystal Tower, which is required now anyway, it would be super awkward to have a whole main storyline where you go to liberated Dalmasca without actually doing the questline that destroys most of the IVth legion and leads to the liberation of Dalmasca. You can’t just have “nods” to your completion of the quests when Valnain and the rest of the country is only free (maybe) of its imperial overlord Gabranth only because of Save the Queen and nothing to do with the Garleans in Endwalker.
Dalmasca is in Othard, but Ilsabard is a big place and we can still go there without Dalmasca and a possible post-liberation side story in Dalmasca if you completed Bozja would make sense if we did as well and hopefully lead to SE streamlining requirements for the 3 instances in it that give people trouble.
