The other problem with an ISD is the sheer firepower and durability it has. It would be completely unbalanced or if balanced would be ridiculous to call it an ISD.
24 proton torpedoes punching through in two volleys normally takes one down in books, and snubfighters are too fast for it's guns to handle. It'll basically sit on the table and eat '90s and '75s while snubfighters launch missiles into it's guts and blow it to pieces
Which would actually be another concern of mine. The game didn't model torpedoes in a way that makes sense for capital ship fights, it modeled them in a way that makes sense for dogfights. PTs are dangerous in the books precisely because they ignore shields, which isn't something that is modeled in the game at all, because the game wasn't written to deal with that scale.
Any game has a scale it is optimal for and points at which it breaks. X-Wing doesn't do a great job simulating individual, super skilled pilots going against each other, the small scale damage and effects that contribute to such a fight, the specific equipment and maneuvering profiles of the ships, and the complex decisions, skills, and maneuvering that would entail. It does well simulating small groups of dogfighters and how they interact with each other.
Similarly, it doesn't do well simulating extremely large engagements, or even particularly large ships. The rules weren't built around that.
Which, again, is why I would just assume not see it in this game. The rules just aren't built to simulate the types of tactics and equipment that work against something that size. And the proton torpedo issue is just one of many issues that could be brought up.
I think that, with their "it feels right" scale, the biggest I would want to expect on my table would be a Nebulon B Frigate.
That's about where I am. I could the a Neb-B being a huge centerpiece, seriously threatening the entire board and requiring squadrons worth of star fighters to take down. And a Star Destroyer is something like an order of magnitude more massive and well equipped.