There are some suggestions for story, and some suggestions for content.

Storywise... we kinda don't know what's going to be made important until it becomes important. What might start as a side story in an older expansion might suddenly become hard canon to the MSQ in later expansions. Sometimes the older content isn't effected, you go back and do it at your leisure and the story happens "out of the timeline" for whenever you decided to complete it. Other times side content later becomes mandatory to access other content, or becomes mandatory for the MSQ.

Examples:
"Side Content" that is still optional that you can complete whenever you want out of the timeline
ARR DRG story (meet Estenien and introduction of the Eyes of Nidhogg), Estenien then became super important to HW MSQ, and now Estenien is graduating to a Trust in EW. However, players are not required to ever level DRG to progress MSQ.

ARR Crystal Tower series (meet Graha T'ia and introduction of the Crystal Tower), was completely optional in ARR, HW, SB, and the beginning of ShB. Was changed to a mandatory duty to progress the MSQ in 5.3... furthermore, new players are also now required to complete CT as part of the MSQ in ARR.

SB Return to Ivalice series is still completely optional side content, but it's required to access other optional side content in ShB, Save the Queen story.


Content-wise... there's a lot of "theory crafting" about what content may or may not be more difficult after the stat squish. The dev team is not infallible... although they are being very vocal about the process and are generally confident, mistakes are definitely possible. However, even if something is broken, it won't remain broken forever... but when it will be fixed then becomes an issue. It could be a hotfix, or they might wait for one of the regular patch intervals to address... in a perfect world there won't be any problems and everything will "just work", we'll find out in a couple months.

That being said, a lot of ppl are recommending the completion of older Savage content now if that's something your interested in seeing for the story alterations, or farming for glam. The dev team acknowledged that some things might actually be harder unsync'd after the squish, which is why they're adding an extra echo... but that's also going to be variable in strength depending on the content.

Another big one is the BLU achievements, if you're interested in the Malboro mount... since the echo has to be turned off for those achievements to even proc. If that's unintentionally made more difficult by the squish... then it's gonna require additional rebalancing to the fights or to BLU... which is not going to be a major priority after the launch. If it's broken, I'm confident that eventually they will fix it, but it's definitely something that would probably be rolled into a patch and not addressed with a hotfix.