Sure, different roles are possible. My question is....what roles are people looking for that we don't have yet?
I would argue naval promotions are already much more diverse than land based ones. So what niche is missing from promotions that we could fill?
I get the impression you are arguing that there is no need for change. That's fine. The point of discussion though is to develop new ideas! It's not going to hurt any of us explore the possibilites, and you won't know what can come up with unless you are willing to try.
Here's a starter: most of those promotions you mention are *only available* after 2 or 3 other promotions. The vast majority of my naval units never get past 3 promotions, and even those that do spend most of their lifetime on the lower tiers. A role that is restricted to only veteran units is a role that plays a relatively small part in the dynamics of naval combat, particularly for those who aren't combat experts or who are defensive players. Sure, in late-game things can get much more interesting, but what about before then?
We were talking about submarines before, and there is little effective choice in the promotions you can choose if you want them to be effective. Wolfback is the best, hands down. Now imagine if subs had more options than just Wolfpack 1 -> Wolfpack 2 -> Wolfpack 3. Granted, these lead to other promotions but for most subs that get produced it's pretty samey. Literally every sub I build, for example, will have the same promotions. Similarly, all of my ranged units and all of my melee units will start with the same three promotions because that's the option that allows them to be most versatile (and on a large map with lots of territory relative to the number of units I have, versatility is key).
Imagine for example if ranged naval units could actually be decent scouts. Currently all of my melee units go into the scout role because (again, large map, lots of water) I need that vision and mobility badly. If ranged units were any good at providing vision and mobile responses I could afford to diversify my melee units and have some of them be more aggressive, or tanky, or mess around with things that are more niche.
Those options often aren't even really available though. For the first half of my game, my ships are only going to have 1-2 promotions. Similar to how the first 2-3 ranks for infantary don't do all that much, my roles my naval unit have (and nature of my naval warfare) is pretty samey. I basically have two types of ships - ranged and melee, and that's it. I don't care too much if I loose any because they are replacable, and those built in later eras often start with more experience than those I've managed to keep alive from earlier eras.
Now, that's just me throwing ideas around. I'm trying to give us something to work with though. If this subject is fruitless then it will die out naturally from lack of interest. I think it's worth trying to see if we can do something new though. It's not like we're not at risk of making things less interesting.