So many people here shouting Straw Man of other peoples arguments, saying no, woof, grr at anything that anybody says, regardless ... and for what.
We will get a system that allows a destroyer to attack a city (I guess they also can "melee" attack units in coastal hexes as well, but of course cannot occupy it, but this is a guess). Cities will be harder to take now, a city of equal age, need bombardment and artillery to be taken. Good then that artillery units are stronger against cities (and weaker against units) to make up for that. A destroyer have almost the same strength as a WWI-Infantry. This will mean that a destroyer will do their part in an ambitious assault, but the main attacks will still be by artillery type units and specialist melee units (i.e. marines).
Sure, there will be times when your destroyer can swoop in and take a city that for example another civ has bombarded back to stone age, but how smart is that, any defender standing close to the city will re-take it (and sink your ship) the next turn (a newly taken city usually don't have so many HP) because you have no backup to.
Also, there will be times when you can send for example a flotilla of BB, CV and DD ships and take an island city. But still this will take time (to bombard it down) and still you have just bombarded a city back to stone age. I'm sure, the 60-300 (depending how many ships a destroyer represent) on board marines that exist on destroyers can take over a rubble.
I don't think a destroyer can defend a newly taken city with no other land support. An island city on the other hand is another deal but also not so far fetch.
Edit: What I meant with this post was that I think we all are building up not only a chicken from a feather, but a whole chicken run
