I have seen some developers already tipping their toes into question themes.

What are the limits there? What kind of questions type can be done? What is not possible?

Case #1: Autocompletion question-type

Textfield
When typing autocompletion is offered.
That's it.

The data for autocompletion can be added manually or via CSV-upload.

Everthing is done via GUI. No placing of CSV in the filesystem or URLpointing.

Is that possible via a question theme? Or is the question theme "image selection" already showing the scope and limits of the new concept?

Case #2: Other fields behind certain answer-items.
Offer a way to put a other field behind any answer-item.

Case #3: Randomization of certain answer-items.
Offer a way to select certain answer-items to be randomized.

I'm asking cause I'm remembering the LS 2 (first release before Yii switch) annoucement, where creating your own question type was mentioned for the first time.

#1:
Probably fairly easy with manually inserting the autocomplete items. You would simply add a custom question setting field where the admin could insert a comma-separated list of the items. Uploading the CSV via the GUI may be more difficult (or not possible).

#2:
Not very easy as it would involve more than one question. I think sticking with a JS workaround might be simpler in the end.

#3:
Easy.

Oh, and to answer the question in the topic title - VERY!

tpartner wrote: #2: Not very easy as it would involve more than one question. I think sticking with a JS workaround might be simpler in the end.

Thanks for your impressions working with question themes. So the main limit is the question type to apply the question theme to. You cannot introduce a new question type independent from the ones currently available.

I will take a closer look at the question themes I can find.
Your CardSort seems to be the first ever released questiontheme beside the bundled ImageSelect one.

CardSort
github.com/tpartner/Card-Sorting-3.x

Seems to have the right amount of complexity to learn more about the concept of question themes.
Thanks for releasing.