The other end of the question, is that is the change better than say, just making ZScript far more robust, and implementing the types of things that we could do with AS, using the existing language.
Right! This is the key question. I could focus energies on some of the changes that have been discussed on the forums and in chat (making inter-script communication easier, tracking pointers over time easier, scripting weapons and Link, etc). But if it sounds like transitioning to AngelScript makes more sense, I want to avoid introducing new features that will make cross-compilation harder.

I do not see ZScript as something separate from ZC compatibility issues. If you want to support all the qussts ever made, you need to support all the ZScript ever made with them.
Well, it's true that *ZASM* must be supported. But not necessarily ZScript (we guarantee that old quests will play in new versions; I agree it would be very nice if old quests can be edited in new versions too, but this is not as bedrock of a principle IMO.)

Let me ask you this: is there any reason to support ZScript, *other* than backwards compatibility of old scripts? If there were a ZScript cross-compiler, would this be sufficient? Or is there a fundamental benefit to supporting both languages?

There was a branch with his enemy code, where the internal enemy code was replaced with AS scripting stuff. I recall that he purged it because it was some kind of unworkable nightmare for him, at the time, but I could be mistaken. It would be nice if he documented what he tried to do, what worked, and what did not.
Agreed. Taking stock of past lessons is a key first step to planning for how, and if, to transition to ZScript.