Textastic 9.8.7 now uses MultiMarkdown 6 and supports file transclusion: 

Use {{some_other_file.md}} in a Markdown file to include other Markdown files.

See https://fletcher.github.io/MultiMarkdown-6/syntax/transclusion.html for details.

Ah, I see. Yes, you can add your own templates and also modify the built-in ones.

The templates are strings in a .json file. You can put them into the folder "Local Files/#Textastic/Templates". 

The default HTML template .json file can be found at https://github.com/blach/Textastic-Customization/blob/master/Templates/html.json

You can modify the html.json file and put it into the folder "Local Files/#Textastic/Templates/". As long as you don't change the uuid field in the .json file, it will be used instead of the templates that are built into the app.

This is described at https://github.com/blach/Textastic-Customization/blob/master/README.md#modifying-existing-templates

Please also have a look at this topic in the manual to see how to add the special folder "#Textastic" in Local Files:

https://www.textasticapp.com/v9/manual/customization/custom_syntax_themes_templates.html#adding-custom-templates

Can you please tell me a bit more about what the problem is or what you are trying to do?

You can add custom syntax definitions to Textastic by using TextMate bundles or Sublime Text packages. See
https://www.textasticapp.com/v9/manual/customization/custom_syntax_themes_templates.html

A search for "Z80 Assembly Sublime Text" brought up this Sublime Text package which should work:

https://github.com/mrcook/Z80Assembly

Just put the file "Z80Assembly.tmLanguage.sublime-syntax" into the folder "Local Files/#Textastic/Z80/" and reload customizations. Then you should be able to select "Z80 Assembly" as a syntax definition in the file properties.

Here's how to get the file on an iPad:

• Go to https://github.com/mrcook/Z80Assembly in Safari.
• Tap the green "Code" button and select "Download ZIP".
• Unzip the file in the Files app.
• Copy the file "Z80Assembly.tmLanguage.sublime-syntax" to "On my iPad/Textastic/#Textastic/Z80/".

Thanks, especially for the details on how other editors do it. I'll consider it for a future update.

Thanks for letting me know, I'll fix it in an update.

This already works. Select Textastic -> Customizations -> Show Customization Folder from the main menu and put sublime syntax files in a subfolder, then reload customizations. This should behave the same as the #Textastic folder in Textastic for iPad and iPhone.

You can add custom fonts. This is explained in the manual at https://www.textasticapp.com/v9/manual/customization/custom_fonts.html

Not yet, sorry.