Your comments

In theory it's already customizable, but I haven't documented it yet.
Textastic 4.0 is now available on the App Store!
Thanks for your detailed description. I will think about it!
I'm already using libssh2 for sftp, but sending terminal commands and displaying its output is a whole different beast - that's why there are dedicated, not-so-cheap apps for this :)

The putty source code might help if I want to support it because it has a very liberal license that allows commercial usage: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

I'll consider it for a future update.
For programming languages like C++, Objective-C or Java it displays a list of class and function names that you can tap to jump to it.

For HTML it displays a list of id - for example if you have a div-tag with an id attribute.

For CSS it will show a list of all css selectors.

This is all defined in the bundle. For example, the HTML bundle symbol list definition can be found here: https://github.com/textmate/html.tmbundle/blob/master/Preferences/Symbol%20List:%20ID.plist
The new version 4.0 with code completion for HTML and CSS should be available any day now.
Thanks for your input! To support this properly I think this would require full terminal emulation (like vt100). Otherwise you won't see the output of the commands you enter. Not trivial :) - You can currently use apps like iSSH or Prompt to do that. Although of course this requires switching to another app.