That thread name could be confusing. At least mine is literally about what I'm doing for Essentials (which could be all manner of things), whereas yours is probably just about this editor and is therefore non-descriptive. Plus, y'know, mine was here first!
I like this. I've never liked the Editor in Essentials, and if I was an utter code wizard, this is exactly one of the things I'd create. I like the item icon being shown, which I presume is the icon of the item currently being created (if it exists) and not just filler.
If I were you, I'd make it look more like RMXP's Database. That is, have a list of all defined <things> on the left (and an "Add New" at the bottom), and the input boxes on the right. If possible, there would be tabs/buttons along the top for each kind of <thing> so you can flick between editing them. This is for when you can load PBS files, of course.
It would also be nice to have more of the fields be drop-down boxes (i.e. the move, ability and pocket fields, plus whatever others aren't in the screenshots), so that you can only choose from existing <things>. Remember to include a "none" option. Maybe even hide the TM item's move field if the item isn't defined as a TM/HM. Again, this is for when you can load PBS files (but you can still do the pockets now).
The internal names could be filled in automatically, derived from the <thing> names, but are able to be modified if desired. This should probably check the internal names of other <things>, and automatically add a number on the end of the current <thing>'s derived internal name to make sure it doesn't clash. When saving, check for internal name clashes and refuse to save if there are any.
In terms of how you could expand on this, anything currently defined by a PBS file is fair game.