Subs can still be detected by any adjacent unit. sub detection only gives vision on 2+ tile range. You said yourself that subs need some help, and moving detection to a dedicated spotter (which would also be immune to subs) would help in that respect.
This also would address a current problem where the counter to subs unlocks on a different, non-prereq tech for subs, so there is a space of time where subs are undetectable if a player has rushed Penicillin. You might find that moving detection to dirigibles/zeppelins actually helps the AI, because they would have the tools to see subs even if they are slightly behind or at tech parity.
Their primary role would be as spotters. Sub detection is just an extension of that role.
This entire conversation started because people felt like the spotter role is lost when zeppelins transition to paratroopers. If they can only attack cities that doesn't diminish their main role, because Zeppelins aren't frontline units even if they could attack.
I said subs need help but this isn't what I had in mind, especially when it promotes a unit that has no other uses. For a human player, supply is very important and wasting it on a unit that has a sole purpose of spotter just doesn't cut it for me. Currently, the naval battles are already largely a numbers game where the side with the larger navy generally wins. If I build one of these resulting in one fewer ship, then that's a disadvantage for me. The AI has enough supply to waste it but a human player lacks that luxury. I also struggle to see how this helps the AI. If you make the AI build more of this, it weakens them because here's a unit that does absolutely nothing for them in combat, especially if I don't build any submarines. Maybe you can also shed light how this spotter is immune to subs. I'm curious how that code would work. That immunity still won't save the spotter from getting destroyed by naval units that are generally stronger than their land counterpart. You buff its survivability and suddenly this spotter becomes your front line in land battles.
I also don't get why we are giving the AI the tools to counter a sub, which the player rushed to unlock, when the AI is slightly behind or at tech parity. Shouldn't we reward the player or AI that gets ahead in military techs where they can use their tools to defeat their foes? If I'm in Renaissance Era fighting against an AI that unlocked Fusiliers and Field Guns, I expect to have a bad day because I didn't prioritize the right techs to counter it. The game shouldn't give me the tools to neutralize that advantage the AI achieved so I can be on par with the AI in tech.
For spotter role, we also have aircraft. It has been awhile since I last used planes but, if I recall correctly, you can actually reveal some tiles with your Fighters when you perform an action. Unless that has changed, we already have a better spotter that's contributing to the air battle. Once again, we don't need a dedicated spotter.
I was under the impression that people felt the transition was weird. You have a Zeppelin that can hover over Coasts and Mountains which suddenly becomes a land unit that cannot even traverse through Mountains. Sometimes, it felt like Zeppelins were better since they can hide in the mountains to give you vision. If a spotter is needed, we have planes to fill that role and, with what I modded, the Recon units. If we need something to attack cities, we have Artillery. If we need sub detection, we have the destroyer. All of these roles can be done better by other units making a spotter only Zeppelin practically a forced units.
