The fact that external modules would not require crew is a major advantage that far outweighs the inability to armor the module. Just cram lots of shield generators on the core ship, or better yet, attach one or more external shield modules.
I do like the idea but I think it needs some work. Maybe make the modules need crew, but still not able to quarter them, so the core will need extra crew kinda like carriers. This would reduce the efficiency of the system in some cases. Modules likely wouldn't all have the same crew requirement, so your core ships would need to be designed with the biggest module in mind. So when smaller modules are loaded, you have extra crew just twiddling their thumbs and taking up tonnage.
Yeah, that could work. External modules could simply be banned from having shields, which should also reduce the workload from a coding perspective. I actually wanted an external crew requirement, but I wasn't sure if the added depth was worth the utter pain it would be to code. I'm still not sure just how much work this would be, considering that almost everything here would have to be coded in from scratch, or if it would even be worth the effort.
While I like the idea of a more robust tractor system, and it makes sense to me that a single Tractor Beam should have a limit on the amount of tonnage it can control, I am strongly opposed to adding a new, 'special' type of unit with its own special rules. We just got rid of PDCs because of all the problems having a special unit with special rules caused.
To me, the External Attachment System appears to largely be a better version of hangar space & parasite ships. A significantly better version, which seems bad for game balance.
Likewise, drop tanks sound like a way to avoid -- or at least mitigate -- the new refueling rules. I wouldn't want to see anything along those lines until we have plenty of experience with C# Aurora.
I think 'external ordnance' is better handled with box launchers, or tugging along a second ship that's nothing but launchers and fire controls.
Putting a size or tonnage limit on the effectiveness of a single tractor installation makes sense (though it also requires a change to allow more than one tractor per ship). To me, it sounds like everything else you're asking for is significantly -- game un-balancingly -- better than the current systems. That seems like a bad idea.
The EAS is nothing like a hangar. For one, you can't actually attach ships to an EAS. An EAS cannot maintain or repair parasites, it cannot reload box launchers, and the initial versions are also extremely inefficient. External modules are ridiculously vulnerable to fire, and can only be attached at a maintenance facility. A purpose-built ship will always be significantly better than a ship with a couple of external modules tacked on, and will also be much cheaper.
If you're willing to take the cost of ditched drop tanks (which can't exactly be recovered in the middle of a fight), you should be able to have drop tanks. They're expensive, vulnerable, and massively slow ships down. If their reattachment is limited to maintenance facilities, they'll act much like box launchers do now, providing a one-time range boost. Tankers are more cost-effective and reliable, but drop tanks provide flexibility. IMO, the existing thing with a box-launcher-only platform is kinda gamey. If that doesn't break balance, I don't see why this should. The numbers are obviously non-final.
There's also no actual need for special rules. External modules could have crew just like normal ships, and be able to maintain themselves, except they'll have zero deployment time and maintenance life, and when attached to an EAS, their crew will be transferred over to the parent ship. It shouldn't be that different.
The point of n EAS is to add some flexibility to the design process. A small hangar is obviously better for tacking on things like upgraded sensors, but an EAS will allow older designs to be partially refit to perform roles they weren't originally intended for, like adding extra launchers without the sensor burden, or even simply 'padding' contacts to make a frigate look like a battleship. It's more for the civilian side of things, where it can be used to turn cargo ships into impromptu troop transports, or increase the endurance of outdated colony ships, etc. It's there to allow an element of improvisation.
I agree with the tonnage limit on tractor modules, but the problem with that approach, IMO, is that it makes shifting large multi-million ton stations completely unfeasible, which was the intended purpose of the tractor beam. Maybe the limit could be Engine Power based instead of a strict tonnage limit? For example, a base tech Tractor Module could will allow the tug to have 1000 EP per 5000 ton tractor module, and allow it to shift arbitrarily large stations, albeit extremely slowly? Moving ships with engines could also be locked behind a tech.
Even if nothing comes of this, I'd still like it if we could shut-off military engines when a tractor is active, allowing for slow shipping of military ships.