This is actually where I take the most issue with having DLC instead of a third version. The expansions to main story are one of the more important aspects of a third version, and we lose that entirely with this DLC method. Now Sword and Shield's main story, which felt incredably empty and lackluster, will never get it's chance to be redeemed by a third version changing its story around and filling it out better,
No offense, but I feel that more often than not, story expansions on third versions tend to be pretty underwhelming.
Suicune doesn't really add anything to Gen II's barebones story, and the new character Eusine is tied to Suicune, making him just as pointless by association.
Fighting both Team Magma and Aqua was cool in theory, but Game Freak was kinda bad at balancing their appearances. I mean, you battle Maxie three times and Archie once, which is less than you do in Sapphire. The hell's up with that?
The Distortion World was cool visually, but didn't really add much other than a new form for Giratina. Looker and Charon don't really do anything to change the story much. Heck, the latter doesn't do diddly squat during the story.
Ultra Sun and Moon went from a story about Lillie's family issues and her growth as a person to a story where all of Lillie's character development goes into the toilet because an alien space prism decided it didn't want Lusamine as the main antagonist anymore. I'd say more, but I'd like to avoid going on a rant.
And hey, it's not like we can't see old characters in the new locations. Plus, there's always possibility of more DLC to expand on characters.