Wasn't the old hangar system just "this is how many fighters you can have, if you go over this number Bad Things happen?"
If there's one thing bringing back the concept of hangars (they don't even have to exist as an actual value, just a lore thing) would be good for, it's abolishing the concept of fighter burn speed. The values are generally faster than most ships and the Navigation bonuses are identical to that for frigates anyway, so it mostly serves as UI clutter. When it doesn't and a fighter wing ends up slowing the fleet down (this happened to me in SS+ with Claymores), it's very annoying (OMG why can't you just ride in the carrier and go the same speed as everyone else?!)
"What about if an AI fleet only has fighters left?" Then it should just die, like the player fleet already does.
Admittedly, i guess that means they would also not be able to fight in hyperspace-zone battles...
You could just decree that fighters can move about in hyperspace like everyone else, but can't transition between normal space and hyper without a carrier.
Although requiring a carrier (as in "has a flight deck") is probably too punitive and not necessary for lore/theme purposes - I imagine you could just strap the fighters to any random hyper-capable ship for the jump.