Right now, carriers are practically only usable as mobile radars/anti-nukes. While they can theoretically repair aircraft, in practice aircraft are so fragile that it is extremely rare that an aircraft will take damage and survive a fight to retreat.
The solution is simple: reduce carrier build power significantly, but allow them to construct seaplanes directly. These seaplanes will still be weaker than a dedicated T2 air force, but they would still add a significant amount of depth to late-game naval: a naval force that wins the sea can pose an air threat against targets protected against cannon bombardment, and air will be a consistent presence in late-game naval fights even without the support of a team's air player, with players able to choose whether to build fighters to provide anti-air or to build bombers to assault enemy naval/ground forces.