A reunion with the Chargers would make some sense, especially after the Mike Williams reunion didn't work out. We know he's got an excellent rapport with Herbert, and contrary to what some may think, I don't think it'd be overlapping skill sets with McConkey.
I agree he would improve the Chargers WR group, and I agree that he could co-exist just fine with McConkey. Both of them can excel at Z as well as in the slot.
The real issue for the Chargers is they signed Williams to play X, and Allen isn't well suited for that role at this stage of his career. If they sign Allen, Johnston probably gets pushed back to X instead of moving to Z this season. His skill set is much better suited to Z.
Right now, the Chargers 3 WR set on third downs is generally going to be X Harris, Z Johnston, slot McConkey. IMO substituting Allen for Johnston would be an upgrade.
It would also be great if such a reunion led to Allen finishing his career with the Chargers, since he spent so much of his career with the team.