While this "freedom" is interesting, I think a brief timeline would still be interesting. Even if you do every single sidequests in Reborn, not much time can best between those. At the same time, Reborn city is supposed to be pretty big, so I have no idea how long it would take to travel from Obsidia to Beryl, for example. Still, there are some things that can probably be timed though, if we assume the trainer only gets specific pokémons, and moves forward.
At the same time, I think some things might not make sense anyway. How long are you supposed to be locked in Agate, so that Reborn City gets a makeover? That would take more than a few months, I guess.
Basically: A timeline would be interesting, but I can see why it wouldn't work exactly.
Once the game is finished though, we as a community could still look into it. Some kind of non-official timeline maybe~