Idea for a new specialist: Commander.

Global: Your subs can carry twice as many specialists (This stacks if you have multiple Commanders.)
Local: If Commander is on a sub, you will be able to launch other subs from the sub the commander is on. Orders to do this can be scheduled for the future. When launching subs from subs, the normal rules apply for how many specialists will be on the sub newly launched. Subs launched from subs can target a sub with a pirate. If all pirates present on a sub targeting another sub leave, the sub will be completely stationary until the owner of the sub sends it somewhere. Orders to send a stationary sub somewhere can also be scheduled for the future. If the owner of a stationary sub gets eliminated, the stationary sub will turn into a gift and go to the nearest outpost. A stationary sub is a valid target for a pirate. Specialists and drillers leaving is a possible way to make a sub with 0 drillers and no specialists, also a valid target for a pirate. This method of making empty subs can be used to make a shortcut for allies to use (Use the speed bonus when targeting another sub to get over, then land using either a Navigator or having an outpost nearby.) When you have a 0 sub, it will possess a button that says "Disappear" which removes the 0 sub from the game entirely. (If a sub a pirate was targeting disappears, the sub holding the pirate will become stationary). Orders to make a sub disappear can also be scheduled for the future.

Flavour text: Subs don't have to be alone.

I really like the versatility this offers, although I would probably make the double specialist count a local effect as well. Seems balanced enough, and there is not problems I can think of so I'd say this is a good idea.
Edit: This is actually probably a bad idea. It's essentially make a navigator useless, because you could just reroute all but one sub.

I love this idea! I'd make it so the commander can't be on the split-off sub though. In other words, the commander will continue to where he was previously directed. This way, he's not basically a Navigator Plus.

I'm a bit nervous about the double-specialists thing, I feel like it has a high chance of leading to incredibly overpowered combos (imagine a pirate/navigator/thief/thief/thief/commander, or a smuggler/navigator/helmsman/pirate/assassin for around-the-world attacks) but I may be wrong, and perhaps subsequent changes could be made to rebalance those. Perhaps just allow one more specialist globally instead of three (non-stackable), or one more locally (not including commander, so five total including commander).

Also, instead of stationary submarines (which I suspect might cause a bit of headache for the devs, who I bet have been programming with the assumption that all subs are in motion), perhaps they can travel to the nearest outpost.

Super cool idea. I can imagine some awesome maneuvering with this. I'd love to have one of these on defense. Can you imagine a commander/navigator/smuggler just floating in the middle of your territory? Beautiful for reinforcing wherever you'd need!

....no.... This sounds way more complicated than all the other specialists. It allows you to attack on multiple un-targetable course, the global effect allows for HUGELY overpowered combos, and the stationary subs thing doesn't follow the dynamics of the game, which would be to simply have subs return to the nearest outpost. This would be hell to integrate into the game.

I read the first three sentences and thought it would be kinda cool but when I got to "can target other subs" I said no. After that the whole thing got super confusing and I scrolled, saw there was a crap ton more rules to explain and said no. It's way to complicated. I like having a convoy that can send subs off of it but if it can target otherwise Untargetable subs that's crazy talk. Also, way too many rules! It's so complicated! And yeah global subs with 6 specs is a bit crazy too. I'd say it should be local +2 specialists and he has to be on board

I concur with the general assessment that this is just way too complex. Like a floating outpost with navigator ability.
The beauty of this game is the simple skill and strategy.