I love the two main console games (haven't played 0), and honestly the only major feature I'd love to see is the ability to switch between 'lead PCs' at any point, rather than having to complete your starter's whole quest first.
Let me explain.
I think it would be really neat, and completely doable, for you as the player to have multiple parties moving around the map at any given time. The games already have provisions for each character being the 'main', with fully-created chapter 1s. In-setting, each character's 'story' starts just before they meet the current party, but what if, instead of that, each character's story started when the Player chooses?
Example from OT2: Say I start out wanting to have a team composed of the 4 character on the Eastern Continent, and a team composed of the 4 from the Western Continent. Rather than having to collect all 8 early on, struggling to get everyone leveled equally, and having my main be overleveled from never being able to switch out, OR doing one group of 4 entirely and then the other group of four (dropping back down to low-levels after getting used to high-powered party)... I could get the 4 on the Eastern Continent together, play them for a bit, then switch over to the other party and play them for a bit, so they level up relatively equally. And you can have the parties join up at any time by meeting up at the tavern in a town.
I'm sure there are other ways this could be used too, but especially considering none of the cutscenes involve the party composition, just the character it's relevant too, it's totally doable without major overhauls or additional cutscene creation.
Heck, the ability to follow any Traveler's story and switch between them at will, letting the party/ies form, merge, and split organically could be a huge bonus point to a future game! Not the gigantic overload of characters from 0 that made it hard to form bonds with them, just the basic 8 Traveler set up... but more fluid.
Again using 2 as an example, there are multiple towns where specific pairs of travelers could meet up organically as part of their stories, but then afterward, their paths can either split or remain together as the player feels appropriate. (Winterbloom and Montwise are great examples, since so much happens in them!)
I'd love to hear everyone else's thoughts on this. It feels kind of like a natural progression of the main console games to me, a way to give more control and flexibility to the player without overloading the creators with more work.