If it's possible to add Angelsript at whatever pace you want, without dropping support for ZScript, I think that would be fine. But if you drop support for zscript, even if you replace it with a similar language, you are still disenfranchising some of the most active creators. (I wouldn't count myself as such a person, as I'm in the 'disappeared in the 2.10 days and only recently resurfaced' category, but, for my own position in the matter: I've pretty much completed my first full quest, by heavily relying on ZScript. I would not be eager to learn a different language right away; if I'm going to do that kind of thing, I would go off and learn something I can make programs in myself. But I am already working on ideas for a sequel... that rely on my knowledge of zscript to actually function.) Part of what might also happen if ZScript is dropped too hastily, is that many people will simply use 2.5.2 instead of the new version, which undermines the points of your hard work.
So basically, I'd totally be in favor of adding a new language, and even okay with you developing that language while not really expanding ZScript- as long as compatability for ZScript isn't dropped. I think that would be a terrible mistake. Just look at how many of the most popular quests rely on ZScript, you know?
As far as ideas for new features... I'm pretty sure it's already been discussed here, but I still think that splitting up the NPC Script weapon defenses into 10 parts (a separate defense setting for each of the ten LW_SCRIPTs) would be an excellent addition to the enemy editor, because it would make the separate script weapon types more useful, and much more likely to be used. Needing to script a workaround to constantly change NPCD_SCRIPT is silly, and many users don't even bother, or, as I did, encounter bugs in the course of trying to set it up properly. I also think that added the ten EW_SCRIPT weapon types to the default weapons you can set an enemy to have would be a good idea, because this enables users to easily put together enemies in the editor- without ghost- that use new weapon types that they can script global effects for. Lastly, I think one other enhancement the enemy editor could use, that would improve casual usage, is better built-in adjustments for enemy sizes- basically just a means to type in their tile width, tile height, hit width, hit height, draw X offset, and draw Y offset; those are admittedly pretty easy to set with scripts, but I think you'd get non-scripter users potentially making more interesting bosses this way. I don't know if any of these suggestions are really necessary, but they'd be extremely convenient, and reduce the barriers of confusion for those learning the program.
Regarding setting up a dev forum on PureZC... I'm not a terribly active forumgoer myself at either place, but I do think that would greatly expand your user feedback, since this place is definitely sort of a ghost town by comparison (I must admit I only saw this thread because someone else pointed me at it). Another very big problem is Zeldaclassic.com not keeping up with new version releases- this really undermines the point of having that site, doesn't it? I think updating Zeldaclassic.com would be a very good idea for getting more users to notice new versions.... I was using 2.5.1 until August because I had no idea 2.5.2 existed, because it was only posted on the PureZC forums, in a place I missed. Especially since PureZC's own download link also leads to Zeldaclassic.com, instead of the up to date forum posting. So there's sort of some executive/presentation confusion about the whole project....
In any case, I also want to say that 2.50 was an excellent improvement over 2.10. The ability to script new items and enemies is singlehandedly what made me interested again- finally I can have my Cane of Somaria and so many other neat things I wished for as a teenager. Thank you very much for making ZScript in the first place. Also... 2.50 in general, especially 2.5.2, are far, far less buggy and vastly more stable than the old 2.10 or various 1.92 versions. I've experienced almost no crashes myself, on Windows 7, except for when I did silly things like misusing a while loop when I was learning to script.
I look forward to seeing new developments for ZC, and thank you all for your hard work : )