Great idea, thanks.

Which parts of gui do you have in mind? Fonts color and size too?

Sorry, currently this is not possible.

Possible, but what's the advantage?

Input made by virtual keyboard will not work.

When entering the String in the Text Expander definition, try to use another (direct) keyboard. I tried with English and this works for me:
String: one (change to your abbreviation)
Substitute: 一二三四五六七八九十

Text expander in principle work by monitoring keyboard events. Could you try changing your keyboard setting?
Which keyboard, language and input settings do you use?

Great, I was just about to suggest this.

Dear FastKeys users, During last few weeks, FastKeys forum has been transferred to a new domain. All the information, including users, topics, posts etc. has been transferred too, except user passwords (of course). If you are a registered user from the old FastKeys Forum, please proceed as follows: ...

Dear FastKeys users, thanks for visiting new FastKeys forums. During last few weeks we were able to transfer all the information (users, topics, posts etc.) from the old forum to a new domain. It was not an easy task but we hope you will like it. :) Please note that the user passwords have been chan...

When entering Input or selection fields, the regular text expander and shortcut items are disabled to prevent potential conflicts.
Not sure if I understand you question correctly...

Both would work. General definition is the following: Text Expander expands text abbreviations into longer phrases. Auto Complete suggests words or phrases while you type. FastKeys is much more than this as it offers many advanced features and settings which can make the differences narrow. For exam...