As in the title, I think adding abillity to script your custom cards will be greatly appreciated. I don’t know how hard would it be to implement from the technical standpoint, but cards are already scripted by people so it doesn’t seem so hard. Another option may be to create some kind of modular scripting and by that I mean to give cards simple effects that could be included even in checkbox like “piercing battle damage”, “cannot be destroyed by battle” or “add one [string] from your deck to your hand”.
You should read the update log where I talk about plans for next year. Custom card scripts can be used later. You can use manual mode where you can do anything as well including things that are unscriptable.